Como comparar meses em postgresql

Table 9-26 shows the available functions for date/time value processing, with details appearing in the following subsections. Table 9-25 illustrates the behaviors of the basic arithmetic operators (+, *, etc.). For formatting functions, refer to Section 9.8. You should be familiar with the background information on date/time data types from Section 8.5.

All the functions and operators described below that take time or timestamp inputs actually come in two variants: one that takes time with time zone or timestamp with time zone, and one that takes time without time zone or timestamp without time zone. For brevity, these variants are not shown separately. Also, the + and * operators come in commutative pairs (for example both date + integer and integer + date); we show only one of each such pair.

Table 9-25. Date/Time Operators

Table 9-26. Date/Time Functions

In addition to these functions, the SQL OVERLAPS operator is supported:

This expression yields true when two time periods (defined by their endpoints) overlap, false when they do not overlap. The endpoints can be specified as pairs of dates, times, or time stamps; or as a date, time, or time stamp followed by an interval.

EXTRACT (field FROM source)

The extract function retrieves subfields such as year or hour from date/time values. source must be a value expression of type timestamp, time, or interval. (Expressions of type date will be cast to timestamp and can therefore be used as well.) field is an identifier or string that selects what field to extract from the source value. The extract function returns values of type double precision. The following are valid field names:

century

The century

The first century starts at 0001-01-01 00:00:00 AD, although they did not know it at the time. This definition applies to all Gregorian calendar countries. There is no century number 0, you go from -1 to 1. If you disagree with this, please write your complaint to: Pope, Cathedral Saint-Peter of Roma, Vatican.

PostgreSQL releases before 8.0 did not follow the conventional numbering of centuries, but just returned the year field divided by 100.

The day (of the month) field (1 - 31)

The year field divided by 10

The day of the week (0 - 6; Sunday is 0) (for timestamp values only)

Note that extract's day of the week numbering is different from that of the to_char function.

The day of the year (1 - 365/366) (for timestamp values only)

For date and timestamp values, the number of seconds since 1970-01-01 00:00:00-00 (can be negative); for interval values, the total number of seconds in the interval

Here is how you can convert an epoch value back to a time stamp:

The hour field (0 - 23)

The seconds field, including fractional parts, multiplied by 1 000 000. Note that this includes full seconds.

The millennium

Years in the 1900s are in the second millennium. The third millennium starts January 1, 2001.

PostgreSQL releases before 8.0 did not follow the conventional numbering of millennia, but just returned the year field divided by 1000.

The seconds field, including fractional parts, multiplied by 1000. Note that this includes full seconds.

The minutes field (0 - 59)

For timestamp values, the number of the month within the year (1 - 12) ; for interval values the number of months, modulo 12 (0 - 11)

The quarter of the year (1 - 4) that the day is in (for timestamp values only)

The seconds field, including fractional parts (0 - 59)

The time zone offset from UTC, measured in seconds. Positive values correspond to time zones east of UTC, negative values to zones west of UTC.

The hour component of the time zone offset

The minute component of the time zone offset

The number of the week of the year that the day is in. By definition (ISO 8601), the first week of a year contains January 4 of that year. (The ISO-8601 week starts on Monday.) In other words, the first Thursday of a year is in week 1 of that year. (for timestamp values only)

Because of this, it is possible for early January dates to be part of the 52nd or 53rd week of the previous year. For example, 2005-01-01 is part of the 53rd week of year 2004, and 2006-01-01 is part of the 52nd week of year 2005.

The year field. Keep in mind there is no 0 AD, so subtracting BC years from AD years should be done with care.

The extract function is primarily intended for computational processing. For formatting date/time values for display, see Section 9.8.

The date_part function is modeled on the traditional Ingres equivalent to the SQL-standard function extract:

Note that here the field parameter needs to be a string value, not a name. The valid field names for date_part are the same as for extract.

The function date_trunc is conceptually similar to the trunc function for numbers.

source is a value expression of type timestamp or interval. (Values of type date and time are cast automatically, to timestamp or interval respectively.) field selects to which precision to truncate the input value. The return value is of type timestamp or interval with all fields that are less significant than the selected one set to zero (or one, for day and month).

Valid values for field are:

Examples:

The AT TIME ZONE construct allows conversions of time stamps to different time zones. Table 9-27 shows its variants.

Table 9-27. AT TIME ZONE Variants

In these expressions, the desired time zone zone can be specified either as a text string (e.g., 'PST') or as an interval (e.g., INTERVAL '-08:00'). In the text case, the available zone names are those shown in Table B-4. (It would be useful to support the more general names shown in Table B-6, but this is not yet implemented.)

Examples (supposing that the local time zone is PST8PDT):

The first example takes a zone-less time stamp and interprets it as MST time (UTC-7) to produce a UTC time stamp, which is then rotated to PST (UTC-8) for display. The second example takes a time stamp specified in EST (UTC-5) and converts it to local time in MST (UTC-7).

The function timezone(zone, timestamp) is equivalent to the SQL-conforming construct timestamp AT TIME ZONE zone.

The following functions are available to obtain the current date and/or time:

CURRENT_TIME and CURRENT_TIMESTAMP deliver values with time zone; LOCALTIME and LOCALTIMESTAMP deliver values without time zone.

CURRENT_TIME, CURRENT_TIMESTAMP, LOCALTIME, and LOCALTIMESTAMP can optionally be given a precision parameter, which causes the result to be rounded to that many fractional digits in the seconds field. Without a precision parameter, the result is given to the full available precision.

Note: Prior to PostgreSQL 7.2, the precision parameters were unimplemented, and the result was always given in integer seconds.

Some examples:

The function now() is the traditional PostgreSQL equivalent to CURRENT_TIMESTAMP.

There is also the function timeofday(), which for historical reasons returns a text string rather than a timestamp value:

It is important to know that CURRENT_TIMESTAMP and related functions return the start time of the current transaction; their values do not change during the transaction. This is considered a feature: the intent is to allow a single transaction to have a consistent notion of the "current" time, so that multiple modifications within the same transaction bear the same time stamp. timeofday() returns the wall-clock time and does advance during transactions.

Note: Other database systems may advance these values more frequently.

All the date/time data types also accept the special literal value now to specify the current date and time. Thus, the following three all return the same result:

Tip: You do not want to use the third form when specifying a DEFAULT clause while creating a table. The system will convert now to a timestamp as soon as the constant is parsed, so that when the default value is needed, the time of the table creation would be used! The first two forms will not be evaluated until the default value is used, because they are function calls. Thus they will give the desired behavior of defaulting to the time of row insertion.

Page 2

Table 9-28 shows the available functions for date/time value processing, with details appearing in the following subsections. Table 9-27 illustrates the behaviors of the basic arithmetic operators (+, *, etc.). For formatting functions, refer to Section 9.8. You should be familiar with the background information on date/time data types from Section 8.5.

All the functions and operators described below that take time or timestamp inputs actually come in two variants: one that takes time with time zone or timestamp with time zone, and one that takes time without time zone or timestamp without time zone. For brevity, these variants are not shown separately. Also, the + and * operators come in commutative pairs (for example both date + integer and integer + date); we show only one of each such pair.

Table 9-27. Date/Time Operators

Table 9-28. Date/Time Functions

In addition to these functions, the SQL OVERLAPS operator is supported:

This expression yields true when two time periods (defined by their endpoints) overlap, false when they do not overlap. The endpoints can be specified as pairs of dates, times, or time stamps; or as a date, time, or time stamp followed by an interval. When a pair of values is provided, either the start or the end can be written first; OVERLAPS automatically takes the earlier value of the pair as the start. Each time period is considered to represent the half-open interval start <= time < end, unless start and end are equal in which case it represents that single time instant. This means for instance that two time periods with only an endpoint in common do not overlap.

When adding an interval value to (or subtracting an interval value from) a timestamp with time zone value, the days component advances (or decrements) the date of the timestamp with time zone by the indicated number of days. Across daylight saving time changes (with the session time zone set to a time zone that recognizes DST), this means interval '1 day' does not necessarily equal interval '24 hours'. For example, with the session time zone set to CST7CDT, timestamp with time zone '2005-04-02 12:00-07' + interval '1 day' will produce timestamp with time zone '2005-04-03 12:00-06', while adding interval '24 hours' to the same initial timestamp with time zone produces timestamp with time zone '2005-04-03 13:00-06', as there is a change in daylight saving time at 2005-04-03 02:00 in time zone CST7CDT.

Note there can be ambiguity in the months returned by age because different months have a different number of days. PostgreSQL's approach uses the month from the earlier of the two dates when calculating partial months. For example, age('2004-06-01', '2004-04-30') uses April to yield 1 mon 1 day, while using May would yield 1 mon 2 days because May has 31 days, while April has only 30.

EXTRACT(field FROM source)

The extract function retrieves subfields such as year or hour from date/time values. source must be a value expression of type timestamp, time, or interval. (Expressions of type date are cast to timestamp and can therefore be used as well.) field is an identifier or string that selects what field to extract from the source value. The extract function returns values of type double precision. The following are valid field names:

century

The century

The first century starts at 0001-01-01 00:00:00 AD, although they did not know it at the time. This definition applies to all Gregorian calendar countries. There is no century number 0, you go from -1 century to 1 century. If you disagree with this, please write your complaint to: Pope, Cathedral Saint-Peter of Roma, Vatican.

PostgreSQL releases before 8.0 did not follow the conventional numbering of centuries, but just returned the year field divided by 100.

The day (of the month) field (1 - 31)

The year field divided by 10

The day of the week as Sunday (0) to Saturday (6)

Note that extract's day of the week numbering differs from that of the to_char(..., 'D') function.

The day of the year (1 - 365/366)

For date and timestamp values, the number of seconds since 1970-01-01 00:00:00 UTC (can be negative); for interval values, the total number of seconds in the interval

Here is how you can convert an epoch value back to a time stamp:

(The to_timestamp function encapsulates the above conversion.)

The hour field (0 - 23)

The day of the week as Monday (1) to Sunday (7)

This is identical to dow except for Sunday. This matches the ISO 8601 day of the week numbering.

The ISO 8601 week-numbering year that the date falls in (not applicable to intervals)

Each ISO 8601 week-numbering year begins with the Monday of the week containing the 4th of January, so in early January or late December the ISO year may be different from the Gregorian year. See the week field for more information.

This field is not available in PostgreSQL releases prior to 8.3.

The seconds field, including fractional parts, multiplied by 1 000 000; note that this includes full seconds

The millennium

Years in the 1900s are in the second millennium. The third millennium started January 1, 2001.

PostgreSQL releases before 8.0 did not follow the conventional numbering of millennia, but just returned the year field divided by 1000.

The seconds field, including fractional parts, multiplied by 1000. Note that this includes full seconds.

The minutes field (0 - 59)

For timestamp values, the number of the month within the year (1 - 12) ; for interval values the number of months, modulo 12 (0 - 11)

The quarter of the year (1 - 4) that the date is in

The seconds field, including fractional parts (0 - 59)

The time zone offset from UTC, measured in seconds. Positive values correspond to time zones east of UTC, negative values to zones west of UTC.

The hour component of the time zone offset

The minute component of the time zone offset

The number of the ISO 8601 week-numbering week of the year. By definition, ISO weeks start on Mondays and the first week of a year contains January 4 of that year. In other words, the first Thursday of a year is in week 1 of that year.

In the ISO week-numbering system, it is possible for early-January dates to be part of the 52nd or 53rd week of the previous year, and for late-December dates to be part of the first week of the next year. For example, 2005-01-01 is part of the 53rd week of year 2004, and 2006-01-01 is part of the 52nd week of year 2005, while 2012-12-31 is part of the first week of 2013. It's recommended to use the isoyear field together with week to get consistent results.

The year field. Keep in mind there is no 0 AD, so subtracting BC years from AD years should be done with care.

The extract function is primarily intended for computational processing. For formatting date/time values for display, see Section 9.8.

The date_part function is modeled on the traditional Ingres equivalent to the SQL-standard function extract:

Note that here the field parameter needs to be a string value, not a name. The valid field names for date_part are the same as for extract.

The function date_trunc is conceptually similar to the trunc function for numbers.

source is a value expression of type timestamp or interval. (Values of type date and time are cast automatically to timestamp or interval, respectively.) field selects to which precision to truncate the input value. The return value is of type timestamp or interval with all fields that are less significant than the selected one set to zero (or one, for day and month).

Valid values for field are:

Examples:

The AT TIME ZONE construct allows conversions of time stamps to different time zones. Table 9-29 shows its variants.

Table 9-29. AT TIME ZONE Variants

In these expressions, the desired time zone zone can be specified either as a text string (e.g., 'PST') or as an interval (e.g., INTERVAL '-08:00'). In the text case, a time zone name can be specified in any of the ways described in Section 8.5.3.

Examples (assuming the local time zone is PST8PDT):

The first example takes a time stamp without time zone and interprets it as MST time (UTC-7), which is then converted to PST (UTC-8) for display. The second example takes a time stamp specified in EST (UTC-5) and converts it to local time in MST (UTC-7).

The function timezone(zone, timestamp) is equivalent to the SQL-conforming construct timestamp AT TIME ZONE zone.

PostgreSQL provides a number of functions that return values related to the current date and time. These SQL-standard functions all return values based on the start time of the current transaction:

CURRENT_TIME and CURRENT_TIMESTAMP deliver values with time zone; LOCALTIME and LOCALTIMESTAMP deliver values without time zone.

CURRENT_TIME, CURRENT_TIMESTAMP, LOCALTIME, and LOCALTIMESTAMP can optionally take a precision parameter, which causes the result to be rounded to that many fractional digits in the seconds field. Without a precision parameter, the result is given to the full available precision.

Some examples:

Since these functions return the start time of the current transaction, their values do not change during the transaction. This is considered a feature: the intent is to allow a single transaction to have a consistent notion of the "current" time, so that multiple modifications within the same transaction bear the same time stamp.

Note: Other database systems might advance these values more frequently.

PostgreSQL also provides functions that return the start time of the current statement, as well as the actual current time at the instant the function is called. The complete list of non-SQL-standard time functions is:

transaction_timestamp() is equivalent to CURRENT_TIMESTAMP, but is named to clearly reflect what it returns. statement_timestamp() returns the start time of the current statement (more specifically, the time of receipt of the latest command message from the client). statement_timestamp() and transaction_timestamp() return the same value during the first command of a transaction, but might differ during subsequent commands. clock_timestamp() returns the actual current time, and therefore its value changes even within a single SQL command. timeofday() is a historical PostgreSQL function. Like clock_timestamp(), it returns the actual current time, but as a formatted text string rather than a timestamp with time zone value. now() is a traditional PostgreSQL equivalent to transaction_timestamp().

All the date/time data types also accept the special literal value now to specify the current date and time (again, interpreted as the transaction start time). Thus, the following three all return the same result:

Tip: You do not want to use the third form when specifying a DEFAULT clause while creating a table. The system will convert now to a timestamp as soon as the constant is parsed, so that when the default value is needed, the time of the table creation would be used! The first two forms will not be evaluated until the default value is used, because they are function calls. Thus they will give the desired behavior of defaulting to the time of row insertion.

The following function is available to delay execution of the server process:

pg_sleep makes the current session's process sleep until seconds seconds have elapsed. seconds is a value of type double precision, so fractional-second delays can be specified. For example:

Note: The effective resolution of the sleep interval is platform-specific; 0.01 seconds is a common value. The sleep delay will be at least as long as specified. It might be longer depending on factors such as server load.

Page 3

Table 9-30 shows the available functions for date/time value processing, with details appearing in the following subsections. Table 9-29 illustrates the behaviors of the basic arithmetic operators (+, *, etc.). For formatting functions, refer to Section 9.8. You should be familiar with the background information on date/time data types from Section 8.5.

In addition, the usual comparison operators shown in Table 9-1 are available for the date/time types. Dates and timestamps (with or without time zone) are all comparable, while times (with or without time zone) and intervals can only be compared to other values of the same data type. When comparing a timestamp without time zone to a timestamp with time zone, the former value is assumed to be given in the time zone specified by the TimeZone configuration parameter, and is rotated to UTC for comparison to the latter value (which is already in UTC internally). Similarly, a date value is assumed to represent midnight in the TimeZone zone when comparing it to a timestamp.

All the functions and operators described below that take time or timestamp inputs actually come in two variants: one that takes time with time zone or timestamp with time zone, and one that takes time without time zone or timestamp without time zone. For brevity, these variants are not shown separately. Also, the + and * operators come in commutative pairs (for example both date + integer and integer + date); we show only one of each such pair.

Table 9-29. Date/Time Operators

Table 9-30. Date/Time Functions

In addition to these functions, the SQL OVERLAPS operator is supported:

This expression yields true when two time periods (defined by their endpoints) overlap, false when they do not overlap. The endpoints can be specified as pairs of dates, times, or time stamps; or as a date, time, or time stamp followed by an interval. When a pair of values is provided, either the start or the end can be written first; OVERLAPS automatically takes the earlier value of the pair as the start. Each time period is considered to represent the half-open interval start <= time < end, unless start and end are equal in which case it represents that single time instant. This means for instance that two time periods with only an endpoint in common do not overlap.

When adding an interval value to (or subtracting an interval value from) a timestamp with time zone value, the days component advances or decrements the date of the timestamp with time zone by the indicated number of days, keeping the time of day the same. Across daylight saving time changes (when the session time zone is set to a time zone that recognizes DST), this means interval '1 day' does not necessarily equal interval '24 hours'. For example, with the session time zone set to America/Denver:

This happens because an hour was skipped due to a change in daylight saving time at 2005-04-03 02:00:00 in time zone America/Denver.

Note there can be ambiguity in the months field returned by age because different months have different numbers of days. PostgreSQL's approach uses the month from the earlier of the two dates when calculating partial months. For example, age('2004-06-01', '2004-04-30') uses April to yield 1 mon 1 day, while using May would yield 1 mon 2 days because May has 31 days, while April has only 30.

Subtraction of dates and timestamps can also be complex. One conceptually simple way to perform subtraction is to convert each value to a number of seconds using EXTRACT(EPOCH FROM ...), then subtract the results; this produces the number of seconds between the two values. This will adjust for the number of days in each month, timezone changes, and daylight saving time adjustments. Subtraction of date or timestamp values with the "-" operator returns the number of days (24-hours) and hours/minutes/seconds between the values, making the same adjustments. The age function returns years, months, days, and hours/minutes/seconds, performing field-by-field subtraction and then adjusting for negative field values. The following queries illustrate the differences in these approaches. The sample results were produced with timezone = 'US/Eastern'; there is a daylight saving time change between the two dates used:

EXTRACT(field FROM source)

The extract function retrieves subfields such as year or hour from date/time values. source must be a value expression of type timestamp, time, or interval. (Expressions of type date are cast to timestamp and can therefore be used as well.) field is an identifier or string that selects what field to extract from the source value. The extract function returns values of type double precision. The following are valid field names:

century

The century

The first century starts at 0001-01-01 00:00:00 AD, although they did not know it at the time. This definition applies to all Gregorian calendar countries. There is no century number 0, you go from -1 century to 1 century. If you disagree with this, please write your complaint to: Pope, Cathedral Saint-Peter of Roma, Vatican.

For timestamp values, the day (of the month) field (1 - 31) ; for interval values, the number of days

The year field divided by 10

The day of the week as Sunday (0) to Saturday (6)

Note that extract's day of the week numbering differs from that of the to_char(..., 'D') function.

The day of the year (1 - 365/366)

For timestamp with time zone values, the number of seconds since 1970-01-01 00:00:00 UTC (negative for timestamps before that); for date and timestamp values, the nominal number of seconds since 1970-01-01 00:00:00, without regard to timezone or daylight-savings rules; for interval values, the total number of seconds in the interval

You can convert an epoch value back to a timestamp with time zone with to_timestamp:

Beware that applying to_timestamp to an epoch extracted from a date or timestamp value could produce a misleading result: the result will effectively assume that the original value had been given in UTC, which might not be the case.

The hour field (0 - 23)

The day of the week as Monday (1) to Sunday (7)

This is identical to dow except for Sunday. This matches the ISO 8601 day of the week numbering.

The ISO 8601 week-numbering year that the date falls in (not applicable to intervals)

Each ISO 8601 week-numbering year begins with the Monday of the week containing the 4th of January, so in early January or late December the ISO year may be different from the Gregorian year. See the week field for more information.

This field is not available in PostgreSQL releases prior to 8.3.

The Julian Date corresponding to the date or timestamp (not applicable to intervals). Timestamps that are not local midnight result in a fractional value. See Section B.7 for more information.

The seconds field, including fractional parts, multiplied by 1 000 000; note that this includes full seconds

The millennium

Years in the 1900s are in the second millennium. The third millennium started January 1, 2001.

The seconds field, including fractional parts, multiplied by 1000. Note that this includes full seconds.

The minutes field (0 - 59)

For timestamp values, the number of the month within the year (1 - 12) ; for interval values, the number of months, modulo 12 (0 - 11)

The quarter of the year (1 - 4) that the date is in

The seconds field, including fractional parts (0 - 59)

The time zone offset from UTC, measured in seconds. Positive values correspond to time zones east of UTC, negative values to zones west of UTC. (Technically, PostgreSQL does not use UTC because leap seconds are not handled.)

The hour component of the time zone offset

The minute component of the time zone offset

The number of the ISO 8601 week-numbering week of the year. By definition, ISO weeks start on Mondays and the first week of a year contains January 4 of that year. In other words, the first Thursday of a year is in week 1 of that year.

In the ISO week-numbering system, it is possible for early-January dates to be part of the 52nd or 53rd week of the previous year, and for late-December dates to be part of the first week of the next year. For example, 2005-01-01 is part of the 53rd week of year 2004, and 2006-01-01 is part of the 52nd week of year 2005, while 2012-12-31 is part of the first week of 2013. It's recommended to use the isoyear field together with week to get consistent results.

The year field. Keep in mind there is no 0 AD, so subtracting BC years from AD years should be done with care.

Note: When the input value is +/-Infinity, extract returns +/-Infinity for monotonically-increasing fields (epoch, julian, year, isoyear, decade, century, and millennium). For other fields, NULL is returned. PostgreSQL versions before 9.6 returned zero for all cases of infinite input.

The extract function is primarily intended for computational processing. For formatting date/time values for display, see Section 9.8.

The date_part function is modeled on the traditional Ingres equivalent to the SQL-standard function extract:

Note that here the field parameter needs to be a string value, not a name. The valid field names for date_part are the same as for extract.

The function date_trunc is conceptually similar to the trunc function for numbers.

source is a value expression of type timestamp or interval. (Values of type date and time are cast automatically to timestamp or interval, respectively.) field selects to which precision to truncate the input value. The return value is of type timestamp or interval with all fields that are less significant than the selected one set to zero (or one, for day and month).

Valid values for field are:

Examples:

The AT TIME ZONE converts time stamp without time zone to/from time stamp with time zone, and time values to different time zones. Table 9-31 shows its variants.

Table 9-31. AT TIME ZONE Variants

In these expressions, the desired time zone zone can be specified either as a text string (e.g., 'America/Los_Angeles') or as an interval (e.g., INTERVAL '-08:00'). In the text case, a time zone name can be specified in any of the ways described in Section 8.5.3.

Examples (assuming the local time zone is America/Los_Angeles):

The first example adds a time zone to a value that lacks it, and displays the value using the current TimeZone setting. The second example shifts the time stamp with time zone value to the specified time zone, and returns the value without a time zone. This allows storage and display of values different from the current TimeZone setting. The third example converts Tokyo time to Chicago time. Converting time values to other time zones uses the currently active time zone rules since no date is supplied.

The function timezone(zone, timestamp) is equivalent to the SQL-conforming construct timestamp AT TIME ZONE zone.

PostgreSQL provides a number of functions that return values related to the current date and time. These SQL-standard functions all return values based on the start time of the current transaction:

CURRENT_TIME and CURRENT_TIMESTAMP deliver values with time zone; LOCALTIME and LOCALTIMESTAMP deliver values without time zone.

CURRENT_TIME, CURRENT_TIMESTAMP, LOCALTIME, and LOCALTIMESTAMP can optionally take a precision parameter, which causes the result to be rounded to that many fractional digits in the seconds field. Without a precision parameter, the result is given to the full available precision.

Some examples:

Since these functions return the start time of the current transaction, their values do not change during the transaction. This is considered a feature: the intent is to allow a single transaction to have a consistent notion of the "current" time, so that multiple modifications within the same transaction bear the same time stamp.

Note: Other database systems might advance these values more frequently.

PostgreSQL also provides functions that return the start time of the current statement, as well as the actual current time at the instant the function is called. The complete list of non-SQL-standard time functions is:

transaction_timestamp() is equivalent to CURRENT_TIMESTAMP, but is named to clearly reflect what it returns. statement_timestamp() returns the start time of the current statement (more specifically, the time of receipt of the latest command message from the client). statement_timestamp() and transaction_timestamp() return the same value during the first command of a transaction, but might differ during subsequent commands. clock_timestamp() returns the actual current time, and therefore its value changes even within a single SQL command. timeofday() is a historical PostgreSQL function. Like clock_timestamp(), it returns the actual current time, but as a formatted text string rather than a timestamp with time zone value. now() is a traditional PostgreSQL equivalent to transaction_timestamp().

All the date/time data types also accept the special literal value now to specify the current date and time (again, interpreted as the transaction start time). Thus, the following three all return the same result:

Tip: Do not use the third form when specifying a value to be evaluated later, for example in a DEFAULT clause for a table column. The system will convert now to a timestamp as soon as the constant is parsed, so that when the default value is needed, the time of the table creation would be used! The first two forms will not be evaluated until the default value is used, because they are function calls. Thus they will give the desired behavior of defaulting to the time of row insertion. (See also Section 8.5.1.4.)

The following functions are available to delay execution of the server process:

pg_sleep makes the current session's process sleep until seconds seconds have elapsed. seconds is a value of type double precision, so fractional-second delays can be specified. pg_sleep_for is a convenience function for larger sleep times specified as an interval. pg_sleep_until is a convenience function for when a specific wake-up time is desired. For example:

Note: The effective resolution of the sleep interval is platform-specific; 0.01 seconds is a common value. The sleep delay will be at least as long as specified. It might be longer depending on factors such as server load. In particular, pg_sleep_until is not guaranteed to wake up exactly at the specified time, but it will not wake up any earlier.

Page 4

Table 9-28 shows the available functions for date/time value processing, with details appearing in the following subsections. Table 9-27 illustrates the behaviors of the basic arithmetic operators (+, *, etc.). For formatting functions, refer to Section 9.8. You should be familiar with the background information on date/time data types from Section 8.5.

In addition, the usual comparison operators shown in Table 9-1 are available for the date/time types. Dates and timestamps (with or without time zone) are all comparable, while times (with or without time zone) and intervals can only be compared to other values of the same data type. When comparing a timestamp without time zone to a timestamp with time zone, the former value is assumed to be given in the time zone specified by the TimeZone configuration parameter, and is rotated to UTC for comparison to the latter value (which is already in UTC internally). Similarly, a date value is assumed to represent midnight in the TimeZone zone when comparing it to a timestamp.

All the functions and operators described below that take time or timestamp inputs actually come in two variants: one that takes time with time zone or timestamp with time zone, and one that takes time without time zone or timestamp without time zone. For brevity, these variants are not shown separately. Also, the + and * operators come in commutative pairs (for example both date + integer and integer + date); we show only one of each such pair.

Table 9-27. Date/Time Operators

Table 9-28. Date/Time Functions

In addition to these functions, the SQL OVERLAPS operator is supported:

This expression yields true when two time periods (defined by their endpoints) overlap, false when they do not overlap. The endpoints can be specified as pairs of dates, times, or time stamps; or as a date, time, or time stamp followed by an interval. When a pair of values is provided, either the start or the end can be written first; OVERLAPS automatically takes the earlier value of the pair as the start. Each time period is considered to represent the half-open interval start <= time < end, unless start and end are equal in which case it represents that single time instant. This means for instance that two time periods with only an endpoint in common do not overlap.

When adding an interval value to (or subtracting an interval value from) a timestamp with time zone value, the days component advances or decrements the date of the timestamp with time zone by the indicated number of days, keeping the time of day the same. Across daylight saving time changes (when the session time zone is set to a time zone that recognizes DST), this means interval '1 day' does not necessarily equal interval '24 hours'. For example, with the session time zone set to America/Denver:

This happens because an hour was skipped due to a change in daylight saving time at 2005-04-03 02:00:00 in time zone America/Denver.

Note there can be ambiguity in the months field returned by age because different months have different numbers of days. PostgreSQL's approach uses the month from the earlier of the two dates when calculating partial months. For example, age('2004-06-01', '2004-04-30') uses April to yield 1 mon 1 day, while using May would yield 1 mon 2 days because May has 31 days, while April has only 30.

Subtraction of dates and timestamps can also be complex. One conceptually simple way to perform subtraction is to convert each value to a number of seconds using EXTRACT(EPOCH FROM ...), then subtract the results; this produces the number of seconds between the two values. This will adjust for the number of days in each month, timezone changes, and daylight saving time adjustments. Subtraction of date or timestamp values with the "-" operator returns the number of days (24-hours) and hours/minutes/seconds between the values, making the same adjustments. The age function returns years, months, days, and hours/minutes/seconds, performing field-by-field subtraction and then adjusting for negative field values. The following queries illustrate the differences in these approaches. The sample results were produced with timezone = 'US/Eastern'; there is a daylight saving time change between the two dates used:

EXTRACT(field FROM source)

The extract function retrieves subfields such as year or hour from date/time values. source must be a value expression of type timestamp, time, or interval. (Expressions of type date are cast to timestamp and can therefore be used as well.) field is an identifier or string that selects what field to extract from the source value. The extract function returns values of type double precision. The following are valid field names:

century

The century

The first century starts at 0001-01-01 00:00:00 AD, although they did not know it at the time. This definition applies to all Gregorian calendar countries. There is no century number 0, you go from -1 century to 1 century. If you disagree with this, please write your complaint to: Pope, Cathedral Saint-Peter of Roma, Vatican.

For timestamp values, the day (of the month) field (1 - 31) ; for interval values, the number of days

The year field divided by 10

The day of the week as Sunday (0) to Saturday (6)

Note that extract's day of the week numbering differs from that of the to_char(..., 'D') function.

The day of the year (1 - 365/366)

For timestamp with time zone values, the number of seconds since 1970-01-01 00:00:00 UTC (negative for timestamps before that); for date and timestamp values, the nominal number of seconds since 1970-01-01 00:00:00, without regard to timezone or daylight-savings rules; for interval values, the total number of seconds in the interval

You can convert an epoch value back to a timestamp with time zone with to_timestamp:

Beware that applying to_timestamp to an epoch extracted from a date or timestamp value could produce a misleading result: the result will effectively assume that the original value had been given in UTC, which might not be the case.

The hour field (0 - 23)

The day of the week as Monday (1) to Sunday (7)

This is identical to dow except for Sunday. This matches the ISO 8601 day of the week numbering.

The ISO 8601 week-numbering year that the date falls in (not applicable to intervals)

Each ISO 8601 week-numbering year begins with the Monday of the week containing the 4th of January, so in early January or late December the ISO year may be different from the Gregorian year. See the week field for more information.

This field is not available in PostgreSQL releases prior to 8.3.

The seconds field, including fractional parts, multiplied by 1 000 000; note that this includes full seconds

The millennium

Years in the 1900s are in the second millennium. The third millennium started January 1, 2001.

The seconds field, including fractional parts, multiplied by 1000. Note that this includes full seconds.

The minutes field (0 - 59)

For timestamp values, the number of the month within the year (1 - 12) ; for interval values, the number of months, modulo 12 (0 - 11)

The quarter of the year (1 - 4) that the date is in

The seconds field, including fractional parts (0 - 59)

The time zone offset from UTC, measured in seconds. Positive values correspond to time zones east of UTC, negative values to zones west of UTC. (Technically, PostgreSQL uses UT1 because leap seconds are not handled.)

The hour component of the time zone offset

The minute component of the time zone offset

The number of the ISO 8601 week-numbering week of the year. By definition, ISO weeks start on Mondays and the first week of a year contains January 4 of that year. In other words, the first Thursday of a year is in week 1 of that year.

In the ISO week-numbering system, it is possible for early-January dates to be part of the 52nd or 53rd week of the previous year, and for late-December dates to be part of the first week of the next year. For example, 2005-01-01 is part of the 53rd week of year 2004, and 2006-01-01 is part of the 52nd week of year 2005, while 2012-12-31 is part of the first week of 2013. It's recommended to use the isoyear field together with week to get consistent results.

The year field. Keep in mind there is no 0 AD, so subtracting BC years from AD years should be done with care.

The extract function is primarily intended for computational processing. For formatting date/time values for display, see Section 9.8.

The date_part function is modeled on the traditional Ingres equivalent to the SQL-standard function extract:

Note that here the field parameter needs to be a string value, not a name. The valid field names for date_part are the same as for extract.

The function date_trunc is conceptually similar to the trunc function for numbers.

source is a value expression of type timestamp or interval. (Values of type date and time are cast automatically to timestamp or interval, respectively.) field selects to which precision to truncate the input value. The return value is of type timestamp or interval with all fields that are less significant than the selected one set to zero (or one, for day and month).

Valid values for field are:

Examples:

The AT TIME ZONE converts time stamp without time zone to/from time stamp with time zone, and time values to different time zones. Table 9-29 shows its variants.

Table 9-29. AT TIME ZONE Variants

In these expressions, the desired time zone zone can be specified either as a text string (e.g., 'America/Los_Angeles') or as an interval (e.g., INTERVAL '-08:00'). In the text case, a time zone name can be specified in any of the ways described in Section 8.5.3.

Examples (assuming the local time zone is America/Los_Angeles):

The first example adds a time zone to a value that lacks it, and displays the value using the current TimeZone setting. The second example shifts the time stamp with time zone value to the specified time zone, and returns the value without a time zone. This allows storage and display of values different from the current TimeZone setting. The third example converts Tokyo time to Chicago time. Converting time values to other time zones uses the currently active time zone rules since no date is supplied.

The function timezone(zone, timestamp) is equivalent to the SQL-conforming construct timestamp AT TIME ZONE zone.

PostgreSQL provides a number of functions that return values related to the current date and time. These SQL-standard functions all return values based on the start time of the current transaction:

CURRENT_TIME and CURRENT_TIMESTAMP deliver values with time zone; LOCALTIME and LOCALTIMESTAMP deliver values without time zone.

CURRENT_TIME, CURRENT_TIMESTAMP, LOCALTIME, and LOCALTIMESTAMP can optionally take a precision parameter, which causes the result to be rounded to that many fractional digits in the seconds field. Without a precision parameter, the result is given to the full available precision.

Some examples:

Since these functions return the start time of the current transaction, their values do not change during the transaction. This is considered a feature: the intent is to allow a single transaction to have a consistent notion of the "current" time, so that multiple modifications within the same transaction bear the same time stamp.

Note: Other database systems might advance these values more frequently.

PostgreSQL also provides functions that return the start time of the current statement, as well as the actual current time at the instant the function is called. The complete list of non-SQL-standard time functions is:

transaction_timestamp() is equivalent to CURRENT_TIMESTAMP, but is named to clearly reflect what it returns. statement_timestamp() returns the start time of the current statement (more specifically, the time of receipt of the latest command message from the client). statement_timestamp() and transaction_timestamp() return the same value during the first command of a transaction, but might differ during subsequent commands. clock_timestamp() returns the actual current time, and therefore its value changes even within a single SQL command. timeofday() is a historical PostgreSQL function. Like clock_timestamp(), it returns the actual current time, but as a formatted text string rather than a timestamp with time zone value. now() is a traditional PostgreSQL equivalent to transaction_timestamp().

All the date/time data types also accept the special literal value now to specify the current date and time (again, interpreted as the transaction start time). Thus, the following three all return the same result:

Tip: Do not use the third form when specifying a value to be evaluated later, for example in a DEFAULT clause for a table column. The system will convert now to a timestamp as soon as the constant is parsed, so that when the default value is needed, the time of the table creation would be used! The first two forms will not be evaluated until the default value is used, because they are function calls. Thus they will give the desired behavior of defaulting to the time of row insertion. (See also Section 8.5.1.4.)

The following functions are available to delay execution of the server process:

pg_sleep makes the current session's process sleep until seconds seconds have elapsed. seconds is a value of type double precision, so fractional-second delays can be specified. pg_sleep_for is a convenience function for larger sleep times specified as an interval. pg_sleep_until is a convenience function for when a specific wake-up time is desired. For example:

Note: The effective resolution of the sleep interval is platform-specific; 0.01 seconds is a common value. The sleep delay will be at least as long as specified. It might be longer depending on factors such as server load. In particular, pg_sleep_until is not guaranteed to wake up exactly at the specified time, but it will not wake up any earlier.

Page 5

Table 9-28 shows the available functions for date/time value processing, with details appearing in the following subsections. Table 9-27 illustrates the behaviors of the basic arithmetic operators (+, *, etc.). For formatting functions, refer to Section 9.8. You should be familiar with the background information on date/time data types from Section 8.5.

All the functions and operators described below that take time or timestamp inputs actually come in two variants: one that takes time with time zone or timestamp with time zone, and one that takes time without time zone or timestamp without time zone. For brevity, these variants are not shown separately. Also, the + and * operators come in commutative pairs (for example both date + integer and integer + date); we show only one of each such pair.

Table 9-27. Date/Time Operators

Table 9-28. Date/Time Functions

In addition to these functions, the SQL OVERLAPS operator is supported:

This expression yields true when two time periods (defined by their endpoints) overlap, false when they do not overlap. The endpoints can be specified as pairs of dates, times, or time stamps; or as a date, time, or time stamp followed by an interval. When a pair of values is provided, either the start or the end can be written first; OVERLAPS automatically takes the earlier value of the pair as the start. Each time period is considered to represent the half-open interval start <= time < end, unless start and end are equal in which case it represents that single time instant. This means for instance that two time periods with only an endpoint in common do not overlap.

When adding an interval value to (or subtracting an interval value from) a timestamp with time zone value, the days component advances or decrements the date of the timestamp with time zone by the indicated number of days. Across daylight saving time changes (when the session time zone is set to a time zone that recognizes DST), this means interval '1 day' does not necessarily equal interval '24 hours'. For example, with the session time zone set to CST7CDT, timestamp with time zone '2005-04-02 12:00-07' + interval '1 day' will produce timestamp with time zone '2005-04-03 12:00-06', while adding interval '24 hours' to the same initial timestamp with time zone produces timestamp with time zone '2005-04-03 13:00-06', as there is a change in daylight saving time at 2005-04-03 02:00 in time zone CST7CDT.

Note there can be ambiguity in the months field returned by age because different months have different numbers of days. PostgreSQL's approach uses the month from the earlier of the two dates when calculating partial months. For example, age('2004-06-01', '2004-04-30') uses April to yield 1 mon 1 day, while using May would yield 1 mon 2 days because May has 31 days, while April has only 30.

Subtraction of dates and timestamps can also be complex. One conceptually simple way to perform subtraction is to convert each value to a number of seconds using EXTRACT(EPOCH FROM ...), then subtract the results; this produces the number of seconds between the two values. This will adjust for the number of days in each month, timezone changes, and daylight saving time adjustments. Subtraction of date or timestamp values with the "-" operator returns the number of days (24-hours) and hours/minutes/seconds between the values, making the same adjustments. The age function returns years, months, days, and hours/minutes/seconds, performing field-by-field subtraction and then adjusting for negative field values. The following queries illustrate the differences in these approaches. The sample results were produced with timezone = 'US/Eastern'; there is a daylight saving time change between the two dates used:

EXTRACT(field FROM source)

The extract function retrieves subfields such as year or hour from date/time values. source must be a value expression of type timestamp, time, or interval. (Expressions of type date are cast to timestamp and can therefore be used as well.) field is an identifier or string that selects what field to extract from the source value. The extract function returns values of type double precision. The following are valid field names:

century

The century

The first century starts at 0001-01-01 00:00:00 AD, although they did not know it at the time. This definition applies to all Gregorian calendar countries. There is no century number 0, you go from -1 century to 1 century. If you disagree with this, please write your complaint to: Pope, Cathedral Saint-Peter of Roma, Vatican.

For timestamp values, the day (of the month) field (1 - 31) ; for interval values, the number of days

The year field divided by 10

The day of the week as Sunday (0) to Saturday (6)

Note that extract's day of the week numbering differs from that of the to_char(..., 'D') function.

The day of the year (1 - 365/366)

For timestamp with time zone values, the number of seconds since 1970-01-01 00:00:00 UTC (can be negative); for date and timestamp values, the number of seconds since 1970-01-01 00:00:00 local time; for interval values, the total number of seconds in the interval

Here is how you can convert an epoch value back to a time stamp:

(The to_timestamp function encapsulates the above conversion.)

The hour field (0 - 23)

The day of the week as Monday (1) to Sunday (7)

This is identical to dow except for Sunday. This matches the ISO 8601 day of the week numbering.

The ISO 8601 week-numbering year that the date falls in (not applicable to intervals)

Each ISO 8601 week-numbering year begins with the Monday of the week containing the 4th of January, so in early January or late December the ISO year may be different from the Gregorian year. See the week field for more information.

This field is not available in PostgreSQL releases prior to 8.3.

The seconds field, including fractional parts, multiplied by 1 000 000; note that this includes full seconds

The millennium

Years in the 1900s are in the second millennium. The third millennium started January 1, 2001.

The seconds field, including fractional parts, multiplied by 1000. Note that this includes full seconds.

The minutes field (0 - 59)

For timestamp values, the number of the month within the year (1 - 12) ; for interval values, the number of months, modulo 12 (0 - 11)

The quarter of the year (1 - 4) that the date is in

The seconds field, including fractional parts (0 - 59)

The time zone offset from UTC, measured in seconds. Positive values correspond to time zones east of UTC, negative values to zones west of UTC. (Technically, PostgreSQL uses UT1 because leap seconds are not handled.)

The hour component of the time zone offset

The minute component of the time zone offset

The number of the ISO 8601 week-numbering week of the year. By definition, ISO weeks start on Mondays and the first week of a year contains January 4 of that year. In other words, the first Thursday of a year is in week 1 of that year.

In the ISO week-numbering system, it is possible for early-January dates to be part of the 52nd or 53rd week of the previous year, and for late-December dates to be part of the first week of the next year. For example, 2005-01-01 is part of the 53rd week of year 2004, and 2006-01-01 is part of the 52nd week of year 2005, while 2012-12-31 is part of the first week of 2013. It's recommended to use the isoyear field together with week to get consistent results.

The year field. Keep in mind there is no 0 AD, so subtracting BC years from AD years should be done with care.

The extract function is primarily intended for computational processing. For formatting date/time values for display, see Section 9.8.

The date_part function is modeled on the traditional Ingres equivalent to the SQL-standard function extract:

Note that here the field parameter needs to be a string value, not a name. The valid field names for date_part are the same as for extract.

The function date_trunc is conceptually similar to the trunc function for numbers.

source is a value expression of type timestamp or interval. (Values of type date and time are cast automatically to timestamp or interval, respectively.) field selects to which precision to truncate the input value. The return value is of type timestamp or interval with all fields that are less significant than the selected one set to zero (or one, for day and month).

Valid values for field are:

Examples:

The AT TIME ZONE converts time stamp without time zone to/from time stamp with time zone, and time values to different time zones. Table 9-29 shows its variants.

Table 9-29. AT TIME ZONE Variants

In these expressions, the desired time zone zone can be specified either as a text string (e.g., 'America/Los_Angeles') or as an interval (e.g., INTERVAL '-08:00'). In the text case, a time zone name can be specified in any of the ways described in Section 8.5.3.

Examples (assuming the local time zone is America/Los_Angeles):

The first example adds a time zone to a value that lacks it, and displays the value using the current TimeZone setting. The second example shifts the time stamp with time zone value to the specified time zone, and returns the value without a time zone. This allows storage and display of values different from the current TimeZone setting. The third example converts Tokyo time to Chicago time. Converting time values to other time zones uses the currently active time zone rules since no date is supplied.

The function timezone(zone, timestamp) is equivalent to the SQL-conforming construct timestamp AT TIME ZONE zone.

PostgreSQL provides a number of functions that return values related to the current date and time. These SQL-standard functions all return values based on the start time of the current transaction:

CURRENT_TIME and CURRENT_TIMESTAMP deliver values with time zone; LOCALTIME and LOCALTIMESTAMP deliver values without time zone.

CURRENT_TIME, CURRENT_TIMESTAMP, LOCALTIME, and LOCALTIMESTAMP can optionally take a precision parameter, which causes the result to be rounded to that many fractional digits in the seconds field. Without a precision parameter, the result is given to the full available precision.

Some examples:

Since these functions return the start time of the current transaction, their values do not change during the transaction. This is considered a feature: the intent is to allow a single transaction to have a consistent notion of the "current" time, so that multiple modifications within the same transaction bear the same time stamp.

Note: Other database systems might advance these values more frequently.

PostgreSQL also provides functions that return the start time of the current statement, as well as the actual current time at the instant the function is called. The complete list of non-SQL-standard time functions is:

transaction_timestamp() is equivalent to CURRENT_TIMESTAMP, but is named to clearly reflect what it returns. statement_timestamp() returns the start time of the current statement (more specifically, the time of receipt of the latest command message from the client). statement_timestamp() and transaction_timestamp() return the same value during the first command of a transaction, but might differ during subsequent commands. clock_timestamp() returns the actual current time, and therefore its value changes even within a single SQL command. timeofday() is a historical PostgreSQL function. Like clock_timestamp(), it returns the actual current time, but as a formatted text string rather than a timestamp with time zone value. now() is a traditional PostgreSQL equivalent to transaction_timestamp().

All the date/time data types also accept the special literal value now to specify the current date and time (again, interpreted as the transaction start time). Thus, the following three all return the same result:

Tip: You do not want to use the third form when specifying a DEFAULT clause while creating a table. The system will convert now to a timestamp as soon as the constant is parsed, so that when the default value is needed, the time of the table creation would be used! The first two forms will not be evaluated until the default value is used, because they are function calls. Thus they will give the desired behavior of defaulting to the time of row insertion.

The following functions are available to delay execution of the server process:

pg_sleep makes the current session's process sleep until seconds seconds have elapsed. seconds is a value of type double precision, so fractional-second delays can be specified. pg_sleep_for is a convenience function for larger sleep times specified as an interval. pg_sleep_until is a convenience function for when a specific wake-up time is desired. For example:

Note: The effective resolution of the sleep interval is platform-specific; 0.01 seconds is a common value. The sleep delay will be at least as long as specified. It might be longer depending on factors such as server load. In particular, pg_sleep_until is not guaranteed to wake up exactly at the specified time, but it will not wake up any earlier.

Page 6

Table 9-28 shows the available functions for date/time value processing, with details appearing in the following subsections. Table 9-27 illustrates the behaviors of the basic arithmetic operators (+, *, etc.). For formatting functions, refer to Section 9.8. You should be familiar with the background information on date/time data types from Section 8.5.

All the functions and operators described below that take time or timestamp inputs actually come in two variants: one that takes time with time zone or timestamp with time zone, and one that takes time without time zone or timestamp without time zone. For brevity, these variants are not shown separately. Also, the + and * operators come in commutative pairs (for example both date + integer and integer + date); we show only one of each such pair.

Table 9-27. Date/Time Operators

Table 9-28. Date/Time Functions

In addition to these functions, the SQL OVERLAPS operator is supported:

This expression yields true when two time periods (defined by their endpoints) overlap, false when they do not overlap. The endpoints can be specified as pairs of dates, times, or time stamps; or as a date, time, or time stamp followed by an interval. When a pair of values is provided, either the start or the end can be written first; OVERLAPS automatically takes the earlier value of the pair as the start. Each time period is considered to represent the half-open interval start <= time < end, unless start and end are equal in which case it represents that single time instant. This means for instance that two time periods with only an endpoint in common do not overlap.

When adding an interval value to (or subtracting an interval value from) a timestamp with time zone value, the days component advances (or decrements) the date of the timestamp with time zone by the indicated number of days. Across daylight saving time changes (with the session time zone set to a time zone that recognizes DST), this means interval '1 day' does not necessarily equal interval '24 hours'. For example, with the session time zone set to CST7CDT, timestamp with time zone '2005-04-02 12:00-07' + interval '1 day' will produce timestamp with time zone '2005-04-03 12:00-06', while adding interval '24 hours' to the same initial timestamp with time zone produces timestamp with time zone '2005-04-03 13:00-06', as there is a change in daylight saving time at 2005-04-03 02:00 in time zone CST7CDT.

Note there can be ambiguity in the months returned by age because different months have a different number of days. PostgreSQL's approach uses the month from the earlier of the two dates when calculating partial months. For example, age('2004-06-01', '2004-04-30') uses April to yield 1 mon 1 day, while using May would yield 1 mon 2 days because May has 31 days, while April has only 30.

EXTRACT(field FROM source)

The extract function retrieves subfields such as year or hour from date/time values. source must be a value expression of type timestamp, time, or interval. (Expressions of type date are cast to timestamp and can therefore be used as well.) field is an identifier or string that selects what field to extract from the source value. The extract function returns values of type double precision. The following are valid field names:

century

The century

The first century starts at 0001-01-01 00:00:00 AD, although they did not know it at the time. This definition applies to all Gregorian calendar countries. There is no century number 0, you go from -1 century to 1 century. If you disagree with this, please write your complaint to: Pope, Cathedral Saint-Peter of Roma, Vatican.

PostgreSQL releases before 8.0 did not follow the conventional numbering of centuries, but just returned the year field divided by 100.

For timestamp values, the day (of the month) field (1 - 31) ; for interval values, the number of days

The year field divided by 10

The day of the week as Sunday (0) to Saturday (6)

Note that extract's day of the week numbering differs from that of the to_char(..., 'D') function.

The day of the year (1 - 365/366)

For timestamp with time zone values, the number of seconds since 1970-01-01 00:00:00 UTC (can be negative); for date and timestamp values, the number of seconds since 1970-01-01 00:00:00 local time; for interval values, the total number of seconds in the interval

Here is how you can convert an epoch value back to a time stamp:

(The to_timestamp function encapsulates the above conversion.)

The hour field (0 - 23)

The day of the week as Monday (1) to Sunday (7)

This is identical to dow except for Sunday. This matches the ISO 8601 day of the week numbering.

The ISO 8601 week-numbering year that the date falls in (not applicable to intervals)

Each ISO 8601 week-numbering year begins with the Monday of the week containing the 4th of January, so in early January or late December the ISO year may be different from the Gregorian year. See the week field for more information.

This field is not available in PostgreSQL releases prior to 8.3.

The seconds field, including fractional parts, multiplied by 1 000 000; note that this includes full seconds

The millennium

Years in the 1900s are in the second millennium. The third millennium started January 1, 2001.

PostgreSQL releases before 8.0 did not follow the conventional numbering of millennia, but just returned the year field divided by 1000.

The seconds field, including fractional parts, multiplied by 1000. Note that this includes full seconds.

The minutes field (0 - 59)

For timestamp values, the number of the month within the year (1 - 12) ; for interval values, the number of months, modulo 12 (0 - 11)

The quarter of the year (1 - 4) that the date is in

The seconds field, including fractional parts (0 - 59)

The time zone offset from UTC, measured in seconds. Positive values correspond to time zones east of UTC, negative values to zones west of UTC. (Technically, PostgreSQL uses UT1 because leap seconds are not handled.)

The hour component of the time zone offset

The minute component of the time zone offset

The number of the ISO 8601 week-numbering week of the year. By definition, ISO weeks start on Mondays and the first week of a year contains January 4 of that year. In other words, the first Thursday of a year is in week 1 of that year.

In the ISO week-numbering system, it is possible for early-January dates to be part of the 52nd or 53rd week of the previous year, and for late-December dates to be part of the first week of the next year. For example, 2005-01-01 is part of the 53rd week of year 2004, and 2006-01-01 is part of the 52nd week of year 2005, while 2012-12-31 is part of the first week of 2013. It's recommended to use the isoyear field together with week to get consistent results.

The year field. Keep in mind there is no 0 AD, so subtracting BC years from AD years should be done with care.

The extract function is primarily intended for computational processing. For formatting date/time values for display, see Section 9.8.

The date_part function is modeled on the traditional Ingres equivalent to the SQL-standard function extract:

Note that here the field parameter needs to be a string value, not a name. The valid field names for date_part are the same as for extract.

The function date_trunc is conceptually similar to the trunc function for numbers.

source is a value expression of type timestamp or interval. (Values of type date and time are cast automatically to timestamp or interval, respectively.) field selects to which precision to truncate the input value. The return value is of type timestamp or interval with all fields that are less significant than the selected one set to zero (or one, for day and month).

Valid values for field are:

Examples:

The AT TIME ZONE converts time stamp without time zone to/from time stamp with time zone, and time values to different time zones. Table 9-29 shows its variants.

Table 9-29. AT TIME ZONE Variants

In these expressions, the desired time zone zone can be specified either as a text string (e.g., 'America/Los_Angeles') or as an interval (e.g., INTERVAL '-08:00'). In the text case, a time zone name can be specified in any of the ways described in Section 8.5.3.

Examples (assuming the local time zone is America/Los_Angeles):

The first example adds a time zone to a value that lacks it, and displays the value using the current TimeZone setting. The second example shifts the time stamp with time zone value to the specified time zone, and returns the value without a time zone. This allows storage and display of values different from the current TimeZone setting. The third example converts Tokyo time to Chicago time. Converting time values to other time zones uses the currently active time zone rules since no date is supplied.

The function timezone(zone, timestamp) is equivalent to the SQL-conforming construct timestamp AT TIME ZONE zone.

PostgreSQL provides a number of functions that return values related to the current date and time. These SQL-standard functions all return values based on the start time of the current transaction:

CURRENT_TIME and CURRENT_TIMESTAMP deliver values with time zone; LOCALTIME and LOCALTIMESTAMP deliver values without time zone.

CURRENT_TIME, CURRENT_TIMESTAMP, LOCALTIME, and LOCALTIMESTAMP can optionally take a precision parameter, which causes the result to be rounded to that many fractional digits in the seconds field. Without a precision parameter, the result is given to the full available precision.

Some examples:

Since these functions return the start time of the current transaction, their values do not change during the transaction. This is considered a feature: the intent is to allow a single transaction to have a consistent notion of the "current" time, so that multiple modifications within the same transaction bear the same time stamp.

Note: Other database systems might advance these values more frequently.

PostgreSQL also provides functions that return the start time of the current statement, as well as the actual current time at the instant the function is called. The complete list of non-SQL-standard time functions is:

transaction_timestamp() is equivalent to CURRENT_TIMESTAMP, but is named to clearly reflect what it returns. statement_timestamp() returns the start time of the current statement (more specifically, the time of receipt of the latest command message from the client). statement_timestamp() and transaction_timestamp() return the same value during the first command of a transaction, but might differ during subsequent commands. clock_timestamp() returns the actual current time, and therefore its value changes even within a single SQL command. timeofday() is a historical PostgreSQL function. Like clock_timestamp(), it returns the actual current time, but as a formatted text string rather than a timestamp with time zone value. now() is a traditional PostgreSQL equivalent to transaction_timestamp().

All the date/time data types also accept the special literal value now to specify the current date and time (again, interpreted as the transaction start time). Thus, the following three all return the same result:

Tip: You do not want to use the third form when specifying a DEFAULT clause while creating a table. The system will convert now to a timestamp as soon as the constant is parsed, so that when the default value is needed, the time of the table creation would be used! The first two forms will not be evaluated until the default value is used, because they are function calls. Thus they will give the desired behavior of defaulting to the time of row insertion.

The following function is available to delay execution of the server process:

pg_sleep makes the current session's process sleep until seconds seconds have elapsed. seconds is a value of type double precision, so fractional-second delays can be specified. For example:

Note: The effective resolution of the sleep interval is platform-specific; 0.01 seconds is a common value. The sleep delay will be at least as long as specified. It might be longer depending on factors such as server load.

Page 7

Table 9-28 shows the available functions for date/time value processing, with details appearing in the following subsections. Table 9-27 illustrates the behaviors of the basic arithmetic operators (+, *, etc.). For formatting functions, refer to Section 9.8. You should be familiar with the background information on date/time data types from Section 8.5.

All the functions and operators described below that take time or timestamp inputs actually come in two variants: one that takes time with time zone or timestamp with time zone, and one that takes time without time zone or timestamp without time zone. For brevity, these variants are not shown separately. Also, the + and * operators come in commutative pairs (for example both date + integer and integer + date); we show only one of each such pair.

Table 9-27. Date/Time Operators

Table 9-28. Date/Time Functions

In addition to these functions, the SQL OVERLAPS operator is supported:

This expression yields true when two time periods (defined by their endpoints) overlap, false when they do not overlap. The endpoints can be specified as pairs of dates, times, or time stamps; or as a date, time, or time stamp followed by an interval. When a pair of values is provided, either the start or the end can be written first; OVERLAPS automatically takes the earlier value of the pair as the start. Each time period is considered to represent the half-open interval start <= time < end, unless start and end are equal in which case it represents that single time instant. This means for instance that two time periods with only an endpoint in common do not overlap.

When adding an interval value to (or subtracting an interval value from) a timestamp with time zone value, the days component advances (or decrements) the date of the timestamp with time zone by the indicated number of days. Across daylight saving time changes (with the session time zone set to a time zone that recognizes DST), this means interval '1 day' does not necessarily equal interval '24 hours'. For example, with the session time zone set to CST7CDT, timestamp with time zone '2005-04-02 12:00-07' + interval '1 day' will produce timestamp with time zone '2005-04-03 12:00-06', while adding interval '24 hours' to the same initial timestamp with time zone produces timestamp with time zone '2005-04-03 13:00-06', as there is a change in daylight saving time at 2005-04-03 02:00 in time zone CST7CDT.

Note there can be ambiguity in the months returned by age because different months have a different number of days. PostgreSQL's approach uses the month from the earlier of the two dates when calculating partial months. For example, age('2004-06-01', '2004-04-30') uses April to yield 1 mon 1 day, while using May would yield 1 mon 2 days because May has 31 days, while April has only 30.

EXTRACT(field FROM source)

The extract function retrieves subfields such as year or hour from date/time values. source must be a value expression of type timestamp, time, or interval. (Expressions of type date are cast to timestamp and can therefore be used as well.) field is an identifier or string that selects what field to extract from the source value. The extract function returns values of type double precision. The following are valid field names:

century

The century

The first century starts at 0001-01-01 00:00:00 AD, although they did not know it at the time. This definition applies to all Gregorian calendar countries. There is no century number 0, you go from -1 century to 1 century. If you disagree with this, please write your complaint to: Pope, Cathedral Saint-Peter of Roma, Vatican.

PostgreSQL releases before 8.0 did not follow the conventional numbering of centuries, but just returned the year field divided by 100.

For timestamp values, the day (of the month) field (1 - 31) ; for interval values, the number of days

The year field divided by 10

The day of the week as Sunday (0) to Saturday (6)

Note that extract's day of the week numbering differs from that of the to_char(..., 'D') function.

The day of the year (1 - 365/366)

For timestamp with time zone values, the number of seconds since 1970-01-01 00:00:00 UTC (can be negative); for date and timestamp values, the number of seconds since 1970-01-01 00:00:00 local time; for interval values, the total number of seconds in the interval

Here is how you can convert an epoch value back to a time stamp:

(The to_timestamp function encapsulates the above conversion.)

The hour field (0 - 23)

The day of the week as Monday (1) to Sunday (7)

This is identical to dow except for Sunday. This matches the ISO 8601 day of the week numbering.

The ISO 8601 week-numbering year that the date falls in (not applicable to intervals)

Each ISO 8601 week-numbering year begins with the Monday of the week containing the 4th of January, so in early January or late December the ISO year may be different from the Gregorian year. See the week field for more information.

This field is not available in PostgreSQL releases prior to 8.3.

The seconds field, including fractional parts, multiplied by 1 000 000; note that this includes full seconds

The millennium

Years in the 1900s are in the second millennium. The third millennium started January 1, 2001.

PostgreSQL releases before 8.0 did not follow the conventional numbering of millennia, but just returned the year field divided by 1000.

The seconds field, including fractional parts, multiplied by 1000. Note that this includes full seconds.

The minutes field (0 - 59)

For timestamp values, the number of the month within the year (1 - 12) ; for interval values, the number of months, modulo 12 (0 - 11)

The quarter of the year (1 - 4) that the date is in

The seconds field, including fractional parts (0 - 59)

The time zone offset from UTC, measured in seconds. Positive values correspond to time zones east of UTC, negative values to zones west of UTC. (Technically, PostgreSQL uses UT1 because leap seconds are not handled.)

The hour component of the time zone offset

The minute component of the time zone offset

The number of the ISO 8601 week-numbering week of the year. By definition, ISO weeks start on Mondays and the first week of a year contains January 4 of that year. In other words, the first Thursday of a year is in week 1 of that year.

In the ISO week-numbering system, it is possible for early-January dates to be part of the 52nd or 53rd week of the previous year, and for late-December dates to be part of the first week of the next year. For example, 2005-01-01 is part of the 53rd week of year 2004, and 2006-01-01 is part of the 52nd week of year 2005, while 2012-12-31 is part of the first week of 2013. It's recommended to use the isoyear field together with week to get consistent results.

The year field. Keep in mind there is no 0 AD, so subtracting BC years from AD years should be done with care.

The extract function is primarily intended for computational processing. For formatting date/time values for display, see Section 9.8.

The date_part function is modeled on the traditional Ingres equivalent to the SQL-standard function extract:

Note that here the field parameter needs to be a string value, not a name. The valid field names for date_part are the same as for extract.

The function date_trunc is conceptually similar to the trunc function for numbers.

source is a value expression of type timestamp or interval. (Values of type date and time are cast automatically to timestamp or interval, respectively.) field selects to which precision to truncate the input value. The return value is of type timestamp or interval with all fields that are less significant than the selected one set to zero (or one, for day and month).

Valid values for field are:

Examples:

The AT TIME ZONE construct allows conversions of time stamps to different time zones. Table 9-29 shows its variants.

Table 9-29. AT TIME ZONE Variants

In these expressions, the desired time zone zone can be specified either as a text string (e.g., 'PST') or as an interval (e.g., INTERVAL '-08:00'). In the text case, a time zone name can be specified in any of the ways described in Section 8.5.3.

Examples (assuming the local time zone is PST8PDT):

The first example takes a time stamp without time zone and interprets it as MST time (UTC-7), which is then converted to PST (UTC-8) for display. The second example takes a time stamp specified in EST (UTC-5) and converts it to local time in MST (UTC-7).

The function timezone(zone, timestamp) is equivalent to the SQL-conforming construct timestamp AT TIME ZONE zone.

PostgreSQL provides a number of functions that return values related to the current date and time. These SQL-standard functions all return values based on the start time of the current transaction:

CURRENT_TIME and CURRENT_TIMESTAMP deliver values with time zone; LOCALTIME and LOCALTIMESTAMP deliver values without time zone.

CURRENT_TIME, CURRENT_TIMESTAMP, LOCALTIME, and LOCALTIMESTAMP can optionally take a precision parameter, which causes the result to be rounded to that many fractional digits in the seconds field. Without a precision parameter, the result is given to the full available precision.

Some examples:

Since these functions return the start time of the current transaction, their values do not change during the transaction. This is considered a feature: the intent is to allow a single transaction to have a consistent notion of the "current" time, so that multiple modifications within the same transaction bear the same time stamp.

Note: Other database systems might advance these values more frequently.

PostgreSQL also provides functions that return the start time of the current statement, as well as the actual current time at the instant the function is called. The complete list of non-SQL-standard time functions is:

transaction_timestamp() is equivalent to CURRENT_TIMESTAMP, but is named to clearly reflect what it returns. statement_timestamp() returns the start time of the current statement (more specifically, the time of receipt of the latest command message from the client). statement_timestamp() and transaction_timestamp() return the same value during the first command of a transaction, but might differ during subsequent commands. clock_timestamp() returns the actual current time, and therefore its value changes even within a single SQL command. timeofday() is a historical PostgreSQL function. Like clock_timestamp(), it returns the actual current time, but as a formatted text string rather than a timestamp with time zone value. now() is a traditional PostgreSQL equivalent to transaction_timestamp().

All the date/time data types also accept the special literal value now to specify the current date and time (again, interpreted as the transaction start time). Thus, the following three all return the same result:

Tip: You do not want to use the third form when specifying a DEFAULT clause while creating a table. The system will convert now to a timestamp as soon as the constant is parsed, so that when the default value is needed, the time of the table creation would be used! The first two forms will not be evaluated until the default value is used, because they are function calls. Thus they will give the desired behavior of defaulting to the time of row insertion.

The following function is available to delay execution of the server process:

pg_sleep makes the current session's process sleep until seconds seconds have elapsed. seconds is a value of type double precision, so fractional-second delays can be specified. For example:

Note: The effective resolution of the sleep interval is platform-specific; 0.01 seconds is a common value. The sleep delay will be at least as long as specified. It might be longer depending on factors such as server load.

Page 8

Table 9-28 shows the available functions for date/time value processing, with details appearing in the following subsections. Table 9-27 illustrates the behaviors of the basic arithmetic operators (+, *, etc.). For formatting functions, refer to Section 9.8. You should be familiar with the background information on date/time data types from Section 8.5.

All the functions and operators described below that take time or timestamp inputs actually come in two variants: one that takes time with time zone or timestamp with time zone, and one that takes time without time zone or timestamp without time zone. For brevity, these variants are not shown separately. Also, the + and * operators come in commutative pairs (for example both date + integer and integer + date); we show only one of each such pair.

Table 9-27. Date/Time Operators

Table 9-28. Date/Time Functions

In addition to these functions, the SQL OVERLAPS operator is supported:

This expression yields true when two time periods (defined by their endpoints) overlap, false when they do not overlap. The endpoints can be specified as pairs of dates, times, or time stamps; or as a date, time, or time stamp followed by an interval. When a pair of values is provided, either the start or the end can be written first; OVERLAPS automatically takes the earlier value of the pair as the start. Each time period is considered to represent the half-open interval start <= time < end, unless start and end are equal in which case it represents that single time instant. This means for instance that two time periods with only an endpoint in common do not overlap.

When adding an interval value to (or subtracting an interval value from) a timestamp with time zone value, the days component advances (or decrements) the date of the timestamp with time zone by the indicated number of days. Across daylight saving time changes (with the session time zone set to a time zone that recognizes DST), this means interval '1 day' does not necessarily equal interval '24 hours'. For example, with the session time zone set to CST7CDT, timestamp with time zone '2005-04-02 12:00-07' + interval '1 day' will produce timestamp with time zone '2005-04-03 12:00-06', while adding interval '24 hours' to the same initial timestamp with time zone produces timestamp with time zone '2005-04-03 13:00-06', as there is a change in daylight saving time at 2005-04-03 02:00 in time zone CST7CDT.

Note there can be ambiguity in the months returned by age because different months have a different number of days. PostgreSQL's approach uses the month from the earlier of the two dates when calculating partial months. For example, age('2004-06-01', '2004-04-30') uses April to yield 1 mon 1 day, while using May would yield 1 mon 2 days because May has 31 days, while April has only 30.

EXTRACT(field FROM source)

The extract function retrieves subfields such as year or hour from date/time values. source must be a value expression of type timestamp, time, or interval. (Expressions of type date are cast to timestamp and can therefore be used as well.) field is an identifier or string that selects what field to extract from the source value. The extract function returns values of type double precision. The following are valid field names:

century

The century

The first century starts at 0001-01-01 00:00:00 AD, although they did not know it at the time. This definition applies to all Gregorian calendar countries. There is no century number 0, you go from -1 century to 1 century. If you disagree with this, please write your complaint to: Pope, Cathedral Saint-Peter of Roma, Vatican.

PostgreSQL releases before 8.0 did not follow the conventional numbering of centuries, but just returned the year field divided by 100.

For timestamp values, the day (of the month) field (1 - 31) ; for interval values, the number of days

The year field divided by 10

The day of the week as Sunday (0) to Saturday (6)

Note that extract's day of the week numbering differs from that of the to_char(..., 'D') function.

The day of the year (1 - 365/366)

For date and timestamp values, the number of seconds since 1970-01-01 00:00:00 UTC (can be negative); for interval values, the total number of seconds in the interval

Here is how you can convert an epoch value back to a time stamp:

(The to_timestamp function encapsulates the above conversion.)

The hour field (0 - 23)

The day of the week as Monday (1) to Sunday (7)

This is identical to dow except for Sunday. This matches the ISO 8601 day of the week numbering.

The ISO 8601 week-numbering year that the date falls in (not applicable to intervals)

Each ISO 8601 week-numbering year begins with the Monday of the week containing the 4th of January, so in early January or late December the ISO year may be different from the Gregorian year. See the week field for more information.

This field is not available in PostgreSQL releases prior to 8.3.

The seconds field, including fractional parts, multiplied by 1 000 000; note that this includes full seconds

The millennium

Years in the 1900s are in the second millennium. The third millennium started January 1, 2001.

PostgreSQL releases before 8.0 did not follow the conventional numbering of millennia, but just returned the year field divided by 1000.

The seconds field, including fractional parts, multiplied by 1000. Note that this includes full seconds.

The minutes field (0 - 59)

For timestamp values, the number of the month within the year (1 - 12) ; for interval values, the number of months, modulo 12 (0 - 11)

The quarter of the year (1 - 4) that the date is in

The seconds field, including fractional parts (0 - 59)

The time zone offset from UTC, measured in seconds. Positive values correspond to time zones east of UTC, negative values to zones west of UTC. (Technically, PostgreSQL uses UT1 because leap seconds are not handled.)

The hour component of the time zone offset

The minute component of the time zone offset

The number of the ISO 8601 week-numbering week of the year. By definition, ISO weeks start on Mondays and the first week of a year contains January 4 of that year. In other words, the first Thursday of a year is in week 1 of that year.

In the ISO week-numbering system, it is possible for early-January dates to be part of the 52nd or 53rd week of the previous year, and for late-December dates to be part of the first week of the next year. For example, 2005-01-01 is part of the 53rd week of year 2004, and 2006-01-01 is part of the 52nd week of year 2005, while 2012-12-31 is part of the first week of 2013. It's recommended to use the isoyear field together with week to get consistent results.

The year field. Keep in mind there is no 0 AD, so subtracting BC years from AD years should be done with care.

The extract function is primarily intended for computational processing. For formatting date/time values for display, see Section 9.8.

The date_part function is modeled on the traditional Ingres equivalent to the SQL-standard function extract:

Note that here the field parameter needs to be a string value, not a name. The valid field names for date_part are the same as for extract.

The function date_trunc is conceptually similar to the trunc function for numbers.

source is a value expression of type timestamp or interval. (Values of type date and time are cast automatically to timestamp or interval, respectively.) field selects to which precision to truncate the input value. The return value is of type timestamp or interval with all fields that are less significant than the selected one set to zero (or one, for day and month).

Valid values for field are:

Examples:

The AT TIME ZONE construct allows conversions of time stamps to different time zones. Table 9-29 shows its variants.

Table 9-29. AT TIME ZONE Variants

In these expressions, the desired time zone zone can be specified either as a text string (e.g., 'PST') or as an interval (e.g., INTERVAL '-08:00'). In the text case, a time zone name can be specified in any of the ways described in Section 8.5.3.

Examples (assuming the local time zone is PST8PDT):

The first example takes a time stamp without time zone and interprets it as MST time (UTC-7), which is then converted to PST (UTC-8) for display. The second example takes a time stamp specified in EST (UTC-5) and converts it to local time in MST (UTC-7).

The function timezone(zone, timestamp) is equivalent to the SQL-conforming construct timestamp AT TIME ZONE zone.

PostgreSQL provides a number of functions that return values related to the current date and time. These SQL-standard functions all return values based on the start time of the current transaction:

CURRENT_TIME and CURRENT_TIMESTAMP deliver values with time zone; LOCALTIME and LOCALTIMESTAMP deliver values without time zone.

CURRENT_TIME, CURRENT_TIMESTAMP, LOCALTIME, and LOCALTIMESTAMP can optionally take a precision parameter, which causes the result to be rounded to that many fractional digits in the seconds field. Without a precision parameter, the result is given to the full available precision.

Some examples:

Since these functions return the start time of the current transaction, their values do not change during the transaction. This is considered a feature: the intent is to allow a single transaction to have a consistent notion of the "current" time, so that multiple modifications within the same transaction bear the same time stamp.

Note: Other database systems might advance these values more frequently.

PostgreSQL also provides functions that return the start time of the current statement, as well as the actual current time at the instant the function is called. The complete list of non-SQL-standard time functions is:

transaction_timestamp() is equivalent to CURRENT_TIMESTAMP, but is named to clearly reflect what it returns. statement_timestamp() returns the start time of the current statement (more specifically, the time of receipt of the latest command message from the client). statement_timestamp() and transaction_timestamp() return the same value during the first command of a transaction, but might differ during subsequent commands. clock_timestamp() returns the actual current time, and therefore its value changes even within a single SQL command. timeofday() is a historical PostgreSQL function. Like clock_timestamp(), it returns the actual current time, but as a formatted text string rather than a timestamp with time zone value. now() is a traditional PostgreSQL equivalent to transaction_timestamp().

All the date/time data types also accept the special literal value now to specify the current date and time (again, interpreted as the transaction start time). Thus, the following three all return the same result:

Tip: You do not want to use the third form when specifying a DEFAULT clause while creating a table. The system will convert now to a timestamp as soon as the constant is parsed, so that when the default value is needed, the time of the table creation would be used! The first two forms will not be evaluated until the default value is used, because they are function calls. Thus they will give the desired behavior of defaulting to the time of row insertion.

The following function is available to delay execution of the server process:

pg_sleep makes the current session's process sleep until seconds seconds have elapsed. seconds is a value of type double precision, so fractional-second delays can be specified. For example:

Note: The effective resolution of the sleep interval is platform-specific; 0.01 seconds is a common value. The sleep delay will be at least as long as specified. It might be longer depending on factors such as server load.

Page 9

Table 9-26 shows the available functions for date/time value processing, with details appearing in the following subsections. Table 9-25 illustrates the behaviors of the basic arithmetic operators (+, *, etc.). For formatting functions, refer to Section 9.8. You should be familiar with the background information on date/time data types from Section 8.5.

All the functions and operators described below that take time or timestamp inputs actually come in two variants: one that takes time with time zone or timestamp with time zone, and one that takes time without time zone or timestamp without time zone. For brevity, these variants are not shown separately. Also, the + and * operators come in commutative pairs (for example both date + integer and integer + date); we show only one of each such pair.

Table 9-25. Date/Time Operators

Table 9-26. Date/Time Functions

If you are using both justify_hours and justify_days, it is best to use justify_hours first so any additional days will be included in the justify_days calculation.

In addition to these functions, the SQL OVERLAPS operator is supported:

This expression yields true when two time periods (defined by their endpoints) overlap, false when they do not overlap. The endpoints can be specified as pairs of dates, times, or time stamps; or as a date, time, or time stamp followed by an interval.

When adding an interval value to (or subtracting an interval value from) a timestamp with time zone value, the days component advances (or decrements) the date of the timestamp with time zone by the indicated number of days. Across daylight saving time changes (with the session time zone set to a time zone that recognizes DST), this means interval '1 day' does not necessarily equal interval '24 hours'. For example, with the session time zone set to CST7CDT, timestamp with time zone '2005-04-02 12:00-07' + interval '1 day' will produce timestamp with time zone '2005-04-03 12:00-06', while adding interval '24 hours' to the same initial timestamp with time zone produces timestamp with time zone '2005-04-03 13:00-06', as there is a change in daylight saving time at 2005-04-03 02:00 in time zone CST7CDT.

EXTRACT(field FROM source)

The extract function retrieves subfields such as year or hour from date/time values. source must be a value expression of type timestamp, time, or interval. (Expressions of type date will be cast to timestamp and can therefore be used as well.) field is an identifier or string that selects what field to extract from the source value. The extract function returns values of type double precision. The following are valid field names:

century

The century

The first century starts at 0001-01-01 00:00:00 AD, although they did not know it at the time. This definition applies to all Gregorian calendar countries. There is no century number 0, you go from -1 to 1. If you disagree with this, please write your complaint to: Pope, Cathedral Saint-Peter of Roma, Vatican.

PostgreSQL releases before 8.0 did not follow the conventional numbering of centuries, but just returned the year field divided by 100.

The day (of the month) field (1 - 31)

The year field divided by 10

The day of the week (0 - 6; Sunday is 0) (for timestamp values only)

Note that extract's day of the week numbering is different from that of the to_char function.

The day of the year (1 - 365/366) (for timestamp values only)

For date and timestamp values, the number of seconds since 1970-01-01 00:00:00-00 (can be negative); for interval values, the total number of seconds in the interval

Here is how you can convert an epoch value back to a time stamp:

The hour field (0 - 23)

The seconds field, including fractional parts, multiplied by 1 000 000. Note that this includes full seconds.

The millennium

Years in the 1900s are in the second millennium. The third millennium starts January 1, 2001.

PostgreSQL releases before 8.0 did not follow the conventional numbering of millennia, but just returned the year field divided by 1000.

The seconds field, including fractional parts, multiplied by 1000. Note that this includes full seconds.

The minutes field (0 - 59)

For timestamp values, the number of the month within the year (1 - 12) ; for interval values the number of months, modulo 12 (0 - 11)

The quarter of the year (1 - 4) that the day is in (for timestamp values only)

The seconds field, including fractional parts (0 - 59)

The time zone offset from UTC, measured in seconds. Positive values correspond to time zones east of UTC, negative values to zones west of UTC.

The hour component of the time zone offset

The minute component of the time zone offset

The number of the week of the year that the day is in. By definition (ISO 8601), the first week of a year contains January 4 of that year. (The ISO-8601 week starts on Monday.) In other words, the first Thursday of a year is in week 1 of that year. (for timestamp values only)

Because of this, it is possible for early January dates to be part of the 52nd or 53rd week of the previous year. For example, 2005-01-01 is part of the 53rd week of year 2004, and 2006-01-01 is part of the 52nd week of year 2005.

The year field. Keep in mind there is no 0 AD, so subtracting BC years from AD years should be done with care.

The extract function is primarily intended for computational processing. For formatting date/time values for display, see Section 9.8.

The date_part function is modeled on the traditional Ingres equivalent to the SQL-standard function extract:

Note that here the field parameter needs to be a string value, not a name. The valid field names for date_part are the same as for extract.

The function date_trunc is conceptually similar to the trunc function for numbers.

source is a value expression of type timestamp or interval. (Values of type date and time are cast automatically, to timestamp or interval respectively.) field selects to which precision to truncate the input value. The return value is of type timestamp or interval with all fields that are less significant than the selected one set to zero (or one, for day and month).

Valid values for field are:

Examples:

The AT TIME ZONE construct allows conversions of time stamps to different time zones. Table 9-27 shows its variants.

Table 9-27. AT TIME ZONE Variants

In these expressions, the desired time zone zone can be specified either as a text string (e.g., 'PST') or as an interval (e.g., INTERVAL '-08:00'). In the text case, the available zone names are those shown in either Table B-6 or Table B-4.

Examples (supposing that the local time zone is PST8PDT):

The first example takes a time stamp without time zone and interprets it as MST time (UTC-7), which is then converted to PST (UTC-8) for display. The second example takes a time stamp specified in EST (UTC-5) and converts it to local time in MST (UTC-7).

The function timezone(zone, timestamp) is equivalent to the SQL-conforming construct timestamp AT TIME ZONE zone.

The following functions are available to obtain the current date and/or time:

CURRENT_TIME and CURRENT_TIMESTAMP deliver values with time zone; LOCALTIME and LOCALTIMESTAMP deliver values without time zone.

CURRENT_TIME, CURRENT_TIMESTAMP, LOCALTIME, and LOCALTIMESTAMP can optionally be given a precision parameter, which causes the result to be rounded to that many fractional digits in the seconds field. Without a precision parameter, the result is given to the full available precision.

Note: Prior to PostgreSQL 7.2, the precision parameters were unimplemented, and the result was always given in integer seconds.

Some examples:

The function now() is the traditional PostgreSQL equivalent to CURRENT_TIMESTAMP.

It is important to know that CURRENT_TIMESTAMP and related functions return the start time of the current transaction; their values do not change during the transaction. This is considered a feature: the intent is to allow a single transaction to have a consistent notion of the "current" time, so that multiple modifications within the same transaction bear the same time stamp.

Note: Other database systems may advance these values more frequently.

There is also the function timeofday() which returns the wall-clock time and advances during transactions. For historical reasons timeofday() returns a text string rather than a timestamp value:

All the date/time data types also accept the special literal value now to specify the current date and time. Thus, the following three all return the same result:

Tip: You do not want to use the third form when specifying a DEFAULT clause while creating a table. The system will convert now to a timestamp as soon as the constant is parsed, so that when the default value is needed, the time of the table creation would be used! The first two forms will not be evaluated until the default value is used, because they are function calls. Thus they will give the desired behavior of defaulting to the time of row insertion.

Page 10

Table 9-27 shows the available functions for date/time value processing, with details appearing in the following subsections. Table 9-26 illustrates the behaviors of the basic arithmetic operators (+, *, etc.). For formatting functions, refer to Section 9.8. You should be familiar with the background information on date/time data types from Section 8.5.

All the functions and operators described below that take time or timestamp inputs actually come in two variants: one that takes time with time zone or timestamp with time zone, and one that takes time without time zone or timestamp without time zone. For brevity, these variants are not shown separately. Also, the + and * operators come in commutative pairs (for example both date + integer and integer + date); we show only one of each such pair.

Table 9-26. Date/Time Operators

Table 9-27. Date/Time Functions

In addition to these functions, the SQL OVERLAPS operator is supported:

This expression yields true when two time periods (defined by their endpoints) overlap, false when they do not overlap. The endpoints can be specified as pairs of dates, times, or time stamps; or as a date, time, or time stamp followed by an interval.

When adding an interval value to (or subtracting an interval value from) a timestamp with time zone value, the days component advances (or decrements) the date of the timestamp with time zone by the indicated number of days. Across daylight saving time changes (with the session time zone set to a time zone that recognizes DST), this means interval '1 day' does not necessarily equal interval '24 hours'. For example, with the session time zone set to CST7CDT, timestamp with time zone '2005-04-02 12:00-07' + interval '1 day' will produce timestamp with time zone '2005-04-03 12:00-06', while adding interval '24 hours' to the same initial timestamp with time zone produces timestamp with time zone '2005-04-03 13:00-06', as there is a change in daylight saving time at 2005-04-03 02:00 in time zone CST7CDT.

Note there can be ambiguity in the months returned by age because different months have a different number of days. PostgreSQL's approach uses the month from the earlier of the two dates when calculating partial months. For example, age('2004-06-01', '2004-04-30') uses April to yield 1 mon 1 day, while using May would yield 1 mon 2 days because May has 31 days, while April has only 30.

EXTRACT(field FROM source)

The extract function retrieves subfields such as year or hour from date/time values. source must be a value expression of type timestamp, time, or interval. (Expressions of type date are cast to timestamp and can therefore be used as well.) field is an identifier or string that selects what field to extract from the source value. The extract function returns values of type double precision. The following are valid field names:

century

The century

The first century starts at 0001-01-01 00:00:00 AD, although they did not know it at the time. This definition applies to all Gregorian calendar countries. There is no century number 0, you go from -1 century to 1 century. If you disagree with this, please write your complaint to: Pope, Cathedral Saint-Peter of Roma, Vatican.

PostgreSQL releases before 8.0 did not follow the conventional numbering of centuries, but just returned the year field divided by 100.

The day (of the month) field (1 - 31)

The year field divided by 10

The day of the week as Sunday(0) to Saturday(6)

Note that extract's day of the week numbering differs from that of the to_char(..., 'D') function.

The day of the year (1 - 365/366)

For date and timestamp values, the number of seconds since 1970-01-01 00:00:00 UTC (can be negative); for interval values, the total number of seconds in the interval

Here is how you can convert an epoch value back to a time stamp:

The hour field (0 - 23)

The day of the week as Monday(1) to Sunday(7)

This is identical to dow except for Sunday. This matches the ISO 8601 day of the week numbering.

The ISO 8601 year that the date falls in (not applicable to intervals)

Each ISO year begins with the Monday of the week containing the 4th of January, so in early January or late December the ISO year may be different from the Gregorian year. See the week field for more information.

This field is not available in PostgreSQL releases prior to 8.3.

The seconds field, including fractional parts, multiplied by 1 000 000; note that this includes full seconds

The millennium

Years in the 1900s are in the second millennium. The third millennium started January 1, 2001.

PostgreSQL releases before 8.0 did not follow the conventional numbering of millennia, but just returned the year field divided by 1000.

The seconds field, including fractional parts, multiplied by 1000. Note that this includes full seconds.

The minutes field (0 - 59)

For timestamp values, the number of the month within the year (1 - 12) ; for interval values the number of months, modulo 12 (0 - 11)

The quarter of the year (1 - 4) that the date is in

The seconds field, including fractional parts (0 - 59)

The time zone offset from UTC, measured in seconds. Positive values correspond to time zones east of UTC, negative values to zones west of UTC.

The hour component of the time zone offset

The minute component of the time zone offset

The number of the week of the year that the day is in. By definition (ISO 8601), weeks start on Mondays and the first week of a year contains January 4 of that year. In other words, the first Thursday of a year is in week 1 of that year.

In the ISO definition, it is possible for early-January dates to be part of the 52nd or 53rd week of the previous year, and for late-December dates to be part of the first week of the next year. For example, 2005-01-01 is part of the 53rd week of year 2004, and 2006-01-01 is part of the 52nd week of year 2005, while 2012-12-31 is part of the first week of 2013. It's recommended to use the isoyear field together with week to get consistent results.

The year field. Keep in mind there is no 0 AD, so subtracting BC years from AD years should be done with care.

The extract function is primarily intended for computational processing. For formatting date/time values for display, see Section 9.8.

The date_part function is modeled on the traditional Ingres equivalent to the SQL-standard function extract:

Note that here the field parameter needs to be a string value, not a name. The valid field names for date_part are the same as for extract.

The function date_trunc is conceptually similar to the trunc function for numbers.

source is a value expression of type timestamp or interval. (Values of type date and time are cast automatically to timestamp or interval, respectively.) field selects to which precision to truncate the input value. The return value is of type timestamp or interval with all fields that are less significant than the selected one set to zero (or one, for day and month).

Valid values for field are:

Examples:

The AT TIME ZONE construct allows conversions of time stamps to different time zones. Table 9-28 shows its variants.

Table 9-28. AT TIME ZONE Variants

In these expressions, the desired time zone zone can be specified either as a text string (e.g., 'PST') or as an interval (e.g., INTERVAL '-08:00'). In the text case, a time zone name can be specified in any of the ways described in Section 8.5.3.

Examples (assuming the local time zone is PST8PDT):

The first example takes a time stamp without time zone and interprets it as MST time (UTC-7), which is then converted to PST (UTC-8) for display. The second example takes a time stamp specified in EST (UTC-5) and converts it to local time in MST (UTC-7).

The function timezone(zone, timestamp) is equivalent to the SQL-conforming construct timestamp AT TIME ZONE zone.

PostgreSQL provides a number of functions that return values related to the current date and time. These SQL-standard functions all return values based on the start time of the current transaction:

CURRENT_TIME and CURRENT_TIMESTAMP deliver values with time zone; LOCALTIME and LOCALTIMESTAMP deliver values without time zone.

CURRENT_TIME, CURRENT_TIMESTAMP, LOCALTIME, and LOCALTIMESTAMP can optionally take a precision parameter, which causes the result to be rounded to that many fractional digits in the seconds field. Without a precision parameter, the result is given to the full available precision.

Some examples:

Since these functions return the start time of the current transaction, their values do not change during the transaction. This is considered a feature: the intent is to allow a single transaction to have a consistent notion of the "current" time, so that multiple modifications within the same transaction bear the same time stamp.

Note: Other database systems might advance these values more frequently.

PostgreSQL also provides functions that return the start time of the current statement, as well as the actual current time at the instant the function is called. The complete list of non-SQL-standard time functions is:

transaction_timestamp() is equivalent to CURRENT_TIMESTAMP, but is named to clearly reflect what it returns. statement_timestamp() returns the start time of the current statement (more specifically, the time of receipt of the latest command message from the client). statement_timestamp() and transaction_timestamp() return the same value during the first command of a transaction, but might differ during subsequent commands. clock_timestamp() returns the actual current time, and therefore its value changes even within a single SQL command. timeofday() is a historical PostgreSQL function. Like clock_timestamp(), it returns the actual current time, but as a formatted text string rather than a timestamp with time zone value. now() is a traditional PostgreSQL equivalent to transaction_timestamp().

All the date/time data types also accept the special literal value now to specify the current date and time (again, interpreted as the transaction start time). Thus, the following three all return the same result:

Tip: You do not want to use the third form when specifying a DEFAULT clause while creating a table. The system will convert now to a timestamp as soon as the constant is parsed, so that when the default value is needed, the time of the table creation would be used! The first two forms will not be evaluated until the default value is used, because they are function calls. Thus they will give the desired behavior of defaulting to the time of row insertion.

The following function is available to delay execution of the server process:

pg_sleep makes the current session's process sleep until seconds seconds have elapsed. seconds is a value of type double precision, so fractional-second delays can be specified. For example:

Note: The effective resolution of the sleep interval is platform-specific; 0.01 seconds is a common value. The sleep delay will be at least as long as specified. It might be longer depending on factors such as server load.

Page 11

Table 9-26 shows the available functions for date/time value processing, with details appearing in the following subsections. Table 9-25 illustrates the behaviors of the basic arithmetic operators (+, *, etc.). For formatting functions, refer to Section 9.8. You should be familiar with the background information on date/time data types from Section 8.5.

All the functions and operators described below that take time or timestamp inputs actually come in two variants: one that takes time with time zone or timestamp with time zone, and one that takes time without time zone or timestamp without time zone. For brevity, these variants are not shown separately. Also, the + and * operators come in commutative pairs (for example both date + integer and integer + date); we show only one of each such pair.

Table 9-25. Date/Time Operators

Table 9-26. Date/Time Functions

In addition to these functions, the SQL OVERLAPS operator is supported:

This expression yields true when two time periods (defined by their endpoints) overlap, false when they do not overlap. The endpoints can be specified as pairs of dates, times, or time stamps; or as a date, time, or time stamp followed by an interval.

When adding an interval value to (or subtracting an interval value from) a timestamp with time zone value, the days component advances (or decrements) the date of the timestamp with time zone by the indicated number of days. Across daylight saving time changes (with the session time zone set to a time zone that recognizes DST), this means interval '1 day' does not necessarily equal interval '24 hours'. For example, with the session time zone set to CST7CDT, timestamp with time zone '2005-04-02 12:00-07' + interval '1 day' will produce timestamp with time zone '2005-04-03 12:00-06', while adding interval '24 hours' to the same initial timestamp with time zone produces timestamp with time zone '2005-04-03 13:00-06', as there is a change in daylight saving time at 2005-04-03 02:00 in time zone CST7CDT.

Note there can be ambiguity in the months returned by age because different months have a different number of days. PostgreSQL's approach uses the month from the earlier of the two dates when calculating partial months. For example, age('2004-06-01', '2004-04-30') uses April to yield 1 mon 1 day, while using May would yield 1 mon 2 days because May has 31 days, while April has only 30.

EXTRACT(field FROM source)

The extract function retrieves subfields such as year or hour from date/time values. source must be a value expression of type timestamp, time, or interval. (Expressions of type date will be cast to timestamp and can therefore be used as well.) field is an identifier or string that selects what field to extract from the source value. The extract function returns values of type double precision. The following are valid field names:

century

The century

The first century starts at 0001-01-01 00:00:00 AD, although they did not know it at the time. This definition applies to all Gregorian calendar countries. There is no century number 0, you go from -1 to 1. If you disagree with this, please write your complaint to: Pope, Cathedral Saint-Peter of Roma, Vatican.

PostgreSQL releases before 8.0 did not follow the conventional numbering of centuries, but just returned the year field divided by 100.

The day (of the month) field (1 - 31)

The year field divided by 10

The day of the week as Sunday(0) to Saturday(6)

Note that extract's day of the week numbering is different from that of the to_char(..., 'D') function.

The day of the year (1 - 365/366)

For date and timestamp values, the number of seconds since 1970-01-01 00:00:00-00 (can be negative); for interval values, the total number of seconds in the interval

Here is how you can convert an epoch value back to a time stamp:

The hour field (0 - 23)

The day of the week as Monday(1) to Sunday(7)

This is identical to dow except for Sunday. This matches the ISO 8601 day of the week numbering.

The ISO 8601 year that the date falls in (not applicable to intervals).

Each ISO year begins with the Monday of the week containing the 4th of January, so in early January or late December the ISO year may be different from the Gregorian year. See the week field for more information.

This field is not available in PostgreSQL releases prior to 8.3.

The seconds field, including fractional parts, multiplied by 1 000 000. Note that this includes full seconds.

The millennium

Years in the 1900s are in the second millennium. The third millennium starts January 1, 2001.

PostgreSQL releases before 8.0 did not follow the conventional numbering of millennia, but just returned the year field divided by 1000.

The seconds field, including fractional parts, multiplied by 1000. Note that this includes full seconds.

The minutes field (0 - 59)

For timestamp values, the number of the month within the year (1 - 12) ; for interval values the number of months, modulo 12 (0 - 11)

The quarter of the year (1 - 4) that the day is in

The seconds field, including fractional parts (0 - 59)

The time zone offset from UTC, measured in seconds. Positive values correspond to time zones east of UTC, negative values to zones west of UTC.

The hour component of the time zone offset

The minute component of the time zone offset

The number of the week of the year that the day is in. By definition (ISO 8601), the first week of a year contains January 4 of that year. (The ISO-8601 week starts on Monday.) In other words, the first Thursday of a year is in week 1 of that year.

Because of this, it is possible for early January dates to be part of the 52nd or 53rd week of the previous year. For example, 2005-01-01 is part of the 53rd week of year 2004, and 2006-01-01 is part of the 52nd week of year 2005.

The year field. Keep in mind there is no 0 AD, so subtracting BC years from AD years should be done with care.

The extract function is primarily intended for computational processing. For formatting date/time values for display, see Section 9.8.

The date_part function is modeled on the traditional Ingres equivalent to the SQL-standard function extract:

Note that here the field parameter needs to be a string value, not a name. The valid field names for date_part are the same as for extract.

The function date_trunc is conceptually similar to the trunc function for numbers.

source is a value expression of type timestamp or interval. (Values of type date and time are cast automatically, to timestamp or interval respectively.) field selects to which precision to truncate the input value. The return value is of type timestamp or interval with all fields that are less significant than the selected one set to zero (or one, for day and month).

Valid values for field are:

Examples:

The AT TIME ZONE construct allows conversions of time stamps to different time zones. Table 9-27 shows its variants.

Table 9-27. AT TIME ZONE Variants

In these expressions, the desired time zone zone can be specified either as a text string (e.g., 'PST') or as an interval (e.g., INTERVAL '-08:00'). In the text case, a time zone name can be specified in any of the ways described in Section 8.5.3.

Examples (supposing that the local time zone is PST8PDT):

The first example takes a time stamp without time zone and interprets it as MST time (UTC-7), which is then converted to PST (UTC-8) for display. The second example takes a time stamp specified in EST (UTC-5) and converts it to local time in MST (UTC-7).

The function timezone(zone, timestamp) is equivalent to the SQL-conforming construct timestamp AT TIME ZONE zone.

PostgreSQL provides a number of functions that return values related to the current date and time. These SQL-standard functions all return values based on the start time of the current transaction:

CURRENT_TIME and CURRENT_TIMESTAMP deliver values with time zone; LOCALTIME and LOCALTIMESTAMP deliver values without time zone.

CURRENT_TIME, CURRENT_TIMESTAMP, LOCALTIME, and LOCALTIMESTAMP can optionally be given a precision parameter, which causes the result to be rounded to that many fractional digits in the seconds field. Without a precision parameter, the result is given to the full available precision.

Some examples:

Since these functions return the start time of the current transaction, their values do not change during the transaction. This is considered a feature: the intent is to allow a single transaction to have a consistent notion of the "current" time, so that multiple modifications within the same transaction bear the same time stamp.

Note: Other database systems might advance these values more frequently.

PostgreSQL also provides functions that return the start time of the current statement, as well as the actual current time at the instant the function is called. The complete list of non-SQL-standard time functions is:

now() is a traditional PostgreSQL equivalent to CURRENT_TIMESTAMP. transaction_timestamp() is likewise equivalent to CURRENT_TIMESTAMP, but is named to clearly reflect what it returns. statement_timestamp() returns the start time of the current statement (more specifically, the time of receipt of the latest command message from the client). statement_timestamp() and transaction_timestamp() return the same value during the first command of a transaction, but might differ during subsequent commands. clock_timestamp() returns the actual current time, and therefore its value changes even within a single SQL command. timeofday() is a historical PostgreSQL function. Like clock_timestamp(), it returns the actual current time, but as a formatted text string rather than a timestamp with time zone value.

All the date/time data types also accept the special literal value now to specify the current date and time (again, interpreted as the transaction start time). Thus, the following three all return the same result:

Tip: You do not want to use the third form when specifying a DEFAULT clause while creating a table. The system will convert now to a timestamp as soon as the constant is parsed, so that when the default value is needed, the time of the table creation would be used! The first two forms will not be evaluated until the default value is used, because they are function calls. Thus they will give the desired behavior of defaulting to the time of row insertion.

The following function is available to delay execution of the server process:

pg_sleep makes the current session's process sleep until seconds seconds have elapsed. seconds is a value of type double precision, so fractional-second delays can be specified. For example:

Note: The effective resolution of the sleep interval is platform-specific; 0.01 seconds is a common value. The sleep delay will be at least as long as specified. It might be longer depending on factors such as server load.

Page 12

Table 9-26 shows the available functions for date/time value processing, with details appearing in the following subsections. Table 9-25 illustrates the behaviors of the basic arithmetic operators (+, *, etc.). For formatting functions, refer to Section 9.8. You should be familiar with the background information on date/time data types from Section 8.5.

All the functions and operators described below that take time or timestamp inputs actually come in two variants: one that takes time with time zone or timestamp with time zone, and one that takes time without time zone or timestamp without time zone. For brevity, these variants are not shown separately. Also, the + and * operators come in commutative pairs (for example both date + integer and integer + date); we show only one of each such pair.

Table 9-25. Date/Time Operators

Table 9-26. Date/Time Functions

In addition to these functions, the SQL OVERLAPS operator is supported:

This expression yields true when two time periods (defined by their endpoints) overlap, false when they do not overlap. The endpoints can be specified as pairs of dates, times, or time stamps; or as a date, time, or time stamp followed by an interval.

When adding an interval value to (or subtracting an interval value from) a timestamp with time zone value, the days component advances (or decrements) the date of the timestamp with time zone by the indicated number of days. Across daylight saving time changes (with the session time zone set to a time zone that recognizes DST), this means interval '1 day' does not necessarily equal interval '24 hours'. For example, with the session time zone set to CST7CDT, timestamp with time zone '2005-04-02 12:00-07' + interval '1 day' will produce timestamp with time zone '2005-04-03 12:00-06', while adding interval '24 hours' to the same initial timestamp with time zone produces timestamp with time zone '2005-04-03 13:00-06', as there is a change in daylight saving time at 2005-04-03 02:00 in time zone CST7CDT.

EXTRACT(field FROM source)

The extract function retrieves subfields such as year or hour from date/time values. source must be a value expression of type timestamp, time, or interval. (Expressions of type date will be cast to timestamp and can therefore be used as well.) field is an identifier or string that selects what field to extract from the source value. The extract function returns values of type double precision. The following are valid field names:

century

The century

The first century starts at 0001-01-01 00:00:00 AD, although they did not know it at the time. This definition applies to all Gregorian calendar countries. There is no century number 0, you go from -1 to 1. If you disagree with this, please write your complaint to: Pope, Cathedral Saint-Peter of Roma, Vatican.

PostgreSQL releases before 8.0 did not follow the conventional numbering of centuries, but just returned the year field divided by 100.

The day (of the month) field (1 - 31)

The year field divided by 10

The day of the week (0 - 6; Sunday is 0) (for timestamp values only)

Note that extract's day of the week numbering is different from that of the to_char function.

The day of the year (1 - 365/366) (for timestamp values only)

For date and timestamp values, the number of seconds since 1970-01-01 00:00:00-00 (can be negative); for interval values, the total number of seconds in the interval

Here is how you can convert an epoch value back to a time stamp:

The hour field (0 - 23)

The seconds field, including fractional parts, multiplied by 1 000 000. Note that this includes full seconds.

The millennium

Years in the 1900s are in the second millennium. The third millennium starts January 1, 2001.

PostgreSQL releases before 8.0 did not follow the conventional numbering of millennia, but just returned the year field divided by 1000.

The seconds field, including fractional parts, multiplied by 1000. Note that this includes full seconds.

The minutes field (0 - 59)

For timestamp values, the number of the month within the year (1 - 12) ; for interval values the number of months, modulo 12 (0 - 11)

The quarter of the year (1 - 4) that the day is in (for timestamp values only)

The seconds field, including fractional parts (0 - 59)

The time zone offset from UTC, measured in seconds. Positive values correspond to time zones east of UTC, negative values to zones west of UTC.

The hour component of the time zone offset

The minute component of the time zone offset

The number of the week of the year that the day is in. By definition (ISO 8601), the first week of a year contains January 4 of that year. (The ISO-8601 week starts on Monday.) In other words, the first Thursday of a year is in week 1 of that year. (for timestamp values only)

Because of this, it is possible for early January dates to be part of the 52nd or 53rd week of the previous year. For example, 2005-01-01 is part of the 53rd week of year 2004, and 2006-01-01 is part of the 52nd week of year 2005.

The year field. Keep in mind there is no 0 AD, so subtracting BC years from AD years should be done with care.

The extract function is primarily intended for computational processing. For formatting date/time values for display, see Section 9.8.

The date_part function is modeled on the traditional Ingres equivalent to the SQL-standard function extract:

Note that here the field parameter needs to be a string value, not a name. The valid field names for date_part are the same as for extract.

The function date_trunc is conceptually similar to the trunc function for numbers.

source is a value expression of type timestamp or interval. (Values of type date and time are cast automatically, to timestamp or interval respectively.) field selects to which precision to truncate the input value. The return value is of type timestamp or interval with all fields that are less significant than the selected one set to zero (or one, for day and month).

Valid values for field are:

Examples:

The AT TIME ZONE construct allows conversions of time stamps to different time zones. Table 9-27 shows its variants.

Table 9-27. AT TIME ZONE Variants

In these expressions, the desired time zone zone can be specified either as a text string (e.g., 'PST') or as an interval (e.g., INTERVAL '-08:00'). In the text case, a time zone name may be specified in any of the ways described in Section 8.5.3.

Examples (supposing that the local time zone is PST8PDT):

The first example takes a time stamp without time zone and interprets it as MST time (UTC-7), which is then converted to PST (UTC-8) for display. The second example takes a time stamp specified in EST (UTC-5) and converts it to local time in MST (UTC-7).

The function timezone(zone, timestamp) is equivalent to the SQL-conforming construct timestamp AT TIME ZONE zone.

PostgreSQL provides a number of functions that return values related to the current date and time. These SQL-standard functions all return values based on the start time of the current transaction:

CURRENT_TIME and CURRENT_TIMESTAMP deliver values with time zone; LOCALTIME and LOCALTIMESTAMP deliver values without time zone.

CURRENT_TIME, CURRENT_TIMESTAMP, LOCALTIME, and LOCALTIMESTAMP can optionally be given a precision parameter, which causes the result to be rounded to that many fractional digits in the seconds field. Without a precision parameter, the result is given to the full available precision.

Some examples:

Since these functions return the start time of the current transaction, their values do not change during the transaction. This is considered a feature: the intent is to allow a single transaction to have a consistent notion of the "current" time, so that multiple modifications within the same transaction bear the same time stamp.

Note: Other database systems may advance these values more frequently.

PostgreSQL also provides functions that return the start time of the current statement, as well as the actual current time at the instant the function is called. The complete list of non-SQL-standard time functions is:

now() is a traditional PostgreSQL equivalent to CURRENT_TIMESTAMP. transaction_timestamp() is likewise equivalent to CURRENT_TIMESTAMP, but is named to clearly reflect what it returns. statement_timestamp() returns the start time of the current statement (more specifically, the time of receipt of the latest command message from the client). statement_timestamp() and transaction_timestamp() return the same value during the first command of a transaction, but may differ during subsequent commands. clock_timestamp() returns the actual current time, and therefore its value changes even within a single SQL command. timeofday() is a historical PostgreSQL function. Like clock_timestamp(), it returns the actual current time, but as a formatted text string rather than a timestamp with time zone value.

All the date/time data types also accept the special literal value now to specify the current date and time (again, interpreted as the transaction start time). Thus, the following three all return the same result:

Tip: You do not want to use the third form when specifying a DEFAULT clause while creating a table. The system will convert now to a timestamp as soon as the constant is parsed, so that when the default value is needed, the time of the table creation would be used! The first two forms will not be evaluated until the default value is used, because they are function calls. Thus they will give the desired behavior of defaulting to the time of row insertion.

The following function is available to delay execution of the server process:

pg_sleep makes the current session's process sleep until seconds seconds have elapsed. seconds is a value of type double precision, so fractional-second delays can be specified. For example:

Note: The effective resolution of the sleep interval is platform-specific; 0.01 seconds is a common value. The sleep delay will be at least as long as specified. It may be longer depending on factors such as server load.

Page 13

Table 4-17 shows the available functions for date/time value processing. Table 4-16 illustrates the behaviors of the basic arithmetic operators (+, *, etc.). For formatting functions, refer to Section 4.7. You should be familiar with the background information on date/time data types (see Section 3.5).

The date/time operators described below behave similarly for types involving time zones as well as those without.

Table 4-16. Date/Time Operators

The date/time functions are summarized below, with additional details in subsequent sections.

Table 4-17. Date/Time Functions

EXTRACT (field FROM source)

The extract function retrieves sub-fields from date/time values, such as year or hour. source is a value expression that evaluates to type timestamp or interval. (Expressions of type date or time will be cast to timestamp and can therefore be used as well.) field is an identifier or string that selects what field to extract from the source value. The extract function returns values of type double precision. The following are valid values:

century

The year field divided by 100

Note that the result for the century field is simply the year field divided by 100, and not the conventional definition which puts most years in the 1900's in the twentieth century.

The day (of the month) field (1 - 31)

The year field divided by 10

The day of the week (0 - 6; Sunday is 0) (for timestamp values only)

The day of the year (1 - 365/366) (for timestamp values only)

For date and timestamp values, the number of seconds since 1970-01-01 00:00:00-00 (Result may be negative.); for interval values, the total number of seconds in the interval

The hour field (0 - 23)

The seconds field, including fractional parts, multiplied by 1 000 000. Note that this includes full seconds.

The year field divided by 1000

Note that the result for the millennium field is simply the year field divided by 1000, and not the conventional definition which puts years in the 1900's in the second millennium.

The seconds field, including fractional parts, multiplied by 1000. Note that this includes full seconds.

The minutes field (0 - 59)

For timestamp values, the number of the month within the year (1 - 12) ; for interval values the number of months, modulo 12 (0 - 11)

The quarter of the year (1 - 4) that the day is in (for timestamp values only)

The seconds field, including fractional parts (0 - 59)

The hour component of the time zone offset.

The minute component of the time zone offset.

From a timestamp value, calculate the number of the week of the year that the day is in. By definition (ISO 8601), the first week of a year contains January 4 of that year. (The ISO week starts on Monday.) In other words, the first Thursday of a year is in week 1 of that year.

The year field

The extract function is primarily intended for computational processing. For formatting date/time values for display, see Section 4.7.

The date_part function is modeled on the traditional Ingres equivalent to the SQL-function extract:

Note that here the field value needs to be a string. The valid field values for date_part are the same as for extract.

The function date_trunc is conceptually similar to the trunc function for numbers.

source is a value expression of type timestamp (values of type date and time are cast automatically). field selects to which precision to truncate the time stamp value. The return value is of type timestamp with all fields that are less than the selected one set to zero (or one, for day and month).

Valid values for field are:

The following functions are available to obtain the current date and/or time:

CURRENT_TIME and CURRENT_TIMESTAMP can optionally be given a precision parameter, which causes the result to be rounded to that many fractional digits. Without a precision parameter, the result is given to full available precision.

Note: Prior to PostgreSQL 7.2, the precision parameters were unimplemented, and the result was always given in integer seconds.

Note: The SQL99 standard requires these functions to be written without any parentheses, unless a precision parameter is given. As of PostgreSQL 7.2, an empty pair of parentheses can be written, but this is deprecated and may be removed in a future release.

The function now() is the traditional PostgreSQL equivalent to CURRENT_TIMESTAMP.

There is also timeofday(), which for historical reasons returns a text string rather than a timestamp value:

It is quite important to realize that CURRENT_TIMESTAMP and related functions all return the time as of the start of the current transaction; their values do not increment while a transaction is running. But timeofday() returns the actual current time.

All the date/time data types also accept the special literal value now to specify the current date and time. Thus, the following three all return the same result:

Note: You do not want to use the third form when specifying a DEFAULT value while creating a table. The system will convert now to a timestamp as soon as the constant is parsed, so that when the default value is needed, the time of the table creation would be used! The first two forms will not be evaluated until the default value is used, because they are function calls. Thus they will give the desired behavior of defaulting to the time of row insertion.

Page 14

Table 9-26 shows the available functions for date/time value processing, with details appearing in the following subsections. Table 9-25 illustrates the behaviors of the basic arithmetic operators (+, *, etc.). For formatting functions, refer to Section 9.7. You should be familiar with the background information on date/time data types from Section 8.5.

All the functions and operators described below that take time or timestamp inputs actually come in two variants: one that takes time with time zone or timestamp with time zone, and one that takes time without time zone or timestamp without time zone. For brevity, these variants are not shown separately.

Table 9-25. Date/Time Operators

Table 9-26. Date/Time Functions

In addition to these functions, the SQL OVERLAPS operator is supported:

This expression yields true when two time periods (defined by their endpoints) overlap, false when they do not overlap. The endpoints can be specified as pairs of dates, times, or time stamps; or as a date, time, or time stamp followed by an interval.

EXTRACT (field FROM source)

The extract function retrieves subfields from date/time values, such as year or hour. source is a value expression that evaluates to type timestamp or interval. (Expressions of type date or time will be cast to timestamp and can therefore be used as well.) field is an identifier or string that selects what field to extract from the source value. The extract function returns values of type double precision. The following are valid field names:

century

The year field divided by 100

Note that the result for the century field is simply the year field divided by 100, and not the conventional definition which puts most years in the 1900's in the twentieth century.

The day (of the month) field (1 - 31)

The year field divided by 10

The day of the week (0 - 6; Sunday is 0) (for timestamp values only)

The day of the year (1 - 365/366) (for timestamp values only)

For date and timestamp values, the number of seconds since 1970-01-01 00:00:00-00 (can be negative); for interval values, the total number of seconds in the interval

The hour field (0 - 23)

The seconds field, including fractional parts, multiplied by 1 000 000. Note that this includes full seconds.

The year field divided by 1000

Note that the result for the millennium field is simply the year field divided by 1000, and not the conventional definition which puts years in the 1900's in the second millennium.

The seconds field, including fractional parts, multiplied by 1000. Note that this includes full seconds.

The minutes field (0 - 59)

For timestamp values, the number of the month within the year (1 - 12) ; for interval values the number of months, modulo 12 (0 - 11)

The quarter of the year (1 - 4) that the day is in (for timestamp values only)

The seconds field, including fractional parts (0 - 59)

The time zone offset from UTC, measured in seconds. Positive values correspond to time zones east of UTC, negative values to zones west of UTC.

The hour component of the time zone offset

The minute component of the time zone offset

The number of the week of the year that the day is in. By definition (ISO 8601), the first week of a year contains January 4 of that year. (The ISO-8601 week starts on Monday.) In other words, the first Thursday of a year is in week 1 of that year. (for timestamp values only)

The year field

The extract function is primarily intended for computational processing. For formatting date/time values for display, see Section 9.7.

The date_part function is modeled on the traditional Ingres equivalent to the SQL-standard function extract:

Note that here the field parameter needs to be a string value, not a name. The valid field names for date_part are the same as for extract.

The function date_trunc is conceptually similar to the trunc function for numbers.

source is a value expression of type timestamp or interval. (Values of type date and time are cast automatically, to timestamp or interval respectively.) field selects to which precision to truncate the input value. The return value is of type timestamp or interval with all fields that are less significant than the selected one set to zero (or one, for day and month).

Valid values for field are:

Examples:

The AT TIME ZONE construct allows conversions of time stamps to different time zones. Table 9-27 shows its variants.

Table 9-27. AT TIME ZONE Variants

In these expressions, the desired time zone zone can be specified either as a text string (e.g., 'PST') or as an interval (e.g., INTERVAL '-08:00').

Examples (supposing that the local time zone is PST8PDT):

The first example takes a zone-less time stamp and interprets it as MST time (UTC-7) to produce a UTC time stamp, which is then rotated to PST (UTC-8) for display. The second example takes a time stamp specified in EST (UTC-5) and converts it to local time in MST (UTC-7).

The function timezone(zone, timestamp) is equivalent to the SQL-conforming construct timestamp AT TIME ZONE zone.

The following functions are available to obtain the current date and/or time:

CURRENT_TIME and CURRENT_TIMESTAMP deliver values with time zone; LOCALTIME and LOCALTIMESTAMP deliver values without time zone.

CURRENT_TIME, CURRENT_TIMESTAMP, LOCALTIME, and LOCALTIMESTAMP can optionally be given a precision parameter, which causes the result to be rounded to that many fractional digits in the seconds field. Without a precision parameter, the result is given to the full available precision.

Note: Prior to PostgreSQL 7.2, the precision parameters were unimplemented, and the result was always given in integer seconds.

Some examples:

The function now() is the traditional PostgreSQL equivalent to CURRENT_TIMESTAMP.

There is also the function timeofday(), which for historical reasons returns a text string rather than a timestamp value:

It is important to know that CURRENT_TIMESTAMP and related functions return the start time of the current transaction; their values do not change during the transaction. This is considered a feature: the intent is to allow a single transaction to have a consistent notion of the "current" time, so that multiple modifications within the same transaction bear the same time stamp. timeofday() returns the wall-clock time and does advance during transactions.

Note: Other database systems may advance these values more frequently.

All the date/time data types also accept the special literal value now to specify the current date and time. Thus, the following three all return the same result:

Note: You do not want to use the third form when specifying a DEFAULT clause while creating a table. The system will convert now to a timestamp as soon as the constant is parsed, so that when the default value is needed, the time of the table creation would be used! The first two forms will not be evaluated until the default value is used, because they are function calls. Thus they will give the desired behavior of defaulting to the time of row insertion.

Page 15

Table 6-18 shows the available functions for date/time value processing, with details appearing in the following subsections. Table 6-17 illustrates the behaviors of the basic arithmetic operators (+, *, etc.). For formatting functions, refer to Section 6.7. You should be familiar with the background information on date/time data types (see Section 5.5).

All the functions and operators described below that take time or timestamp inputs actually come in two variants: one that takes time or timestamp with time zone, and one that takes time or timestamp without time zone. For brevity, these variants are not shown separately.

Table 6-17. Date/Time Operators

Table 6-18. Date/Time Functions

EXTRACT (field FROM source)

The extract function retrieves subfields from date/time values, such as year or hour. source is a value expression that evaluates to type timestamp or interval. (Expressions of type date or time will be cast to timestamp and can therefore be used as well.) field is an identifier or string that selects what field to extract from the source value. The extract function returns values of type double precision. The following are valid values:

century

The year field divided by 100

Note that the result for the century field is simply the year field divided by 100, and not the conventional definition which puts most years in the 1900's in the twentieth century.

The day (of the month) field (1 - 31)

The year field divided by 10

The day of the week (0 - 6; Sunday is 0) (for timestamp values only)

The day of the year (1 - 365/366) (for timestamp values only)

For date and timestamp values, the number of seconds since 1970-01-01 00:00:00-00 (can be negative); for interval values, the total number of seconds in the interval

The hour field (0 - 23)

The seconds field, including fractional parts, multiplied by 1 000 000. Note that this includes full seconds.

The year field divided by 1000

Note that the result for the millennium field is simply the year field divided by 1000, and not the conventional definition which puts years in the 1900's in the second millennium.

The seconds field, including fractional parts, multiplied by 1000. Note that this includes full seconds.

The minutes field (0 - 59)

For timestamp values, the number of the month within the year (1 - 12) ; for interval values the number of months, modulo 12 (0 - 11)

The quarter of the year (1 - 4) that the day is in (for timestamp values only)

The seconds field, including fractional parts (0 - 59)

The hour component of the time zone offset.

The minute component of the time zone offset.

From a timestamp value, calculate the number of the week of the year that the day is in. By definition (ISO 8601), the first week of a year contains January 4 of that year. (The ISO week starts on Monday.) In other words, the first Thursday of a year is in week 1 of that year.

The year field

The extract function is primarily intended for computational processing. For formatting date/time values for display, see Section 6.7.

The date_part function is modeled on the traditional Ingres equivalent to the SQL-standard function extract:

Note that here the field parameter needs to be a string value, not a name. The valid field values for date_part are the same as for extract.

The function date_trunc is conceptually similar to the trunc function for numbers.

source is a value expression of type timestamp (values of type date and time are cast automatically). field selects to which precision to truncate the time stamp value. The return value is of type timestamp with all fields that are less than the selected one set to zero (or one, for day and month).

Valid values for field are:

Examples:

The AT TIME ZONE construct allows conversions of timestamps to different timezones.

Table 6-19. AT TIME ZONE Variants

In these expressions, the desired time zone can be specified either as a text string (e.g., 'PST') or as an interval (e.g., INTERVAL '-08:00').

Examples (supposing that TimeZone is PST8PDT):

The first example takes a zone-less timestamp and interprets it as MST time (GMT-7) to produce a UTC timestamp, which is then rotated to PST (GMT-8) for display. The second example takes a timestamp specified in EST (GMT-5) and converts it to local time in MST (GMT-7).

The function timezone(zone, timestamp) is equivalent to the SQL-compliant construct timestamp AT TIME ZONE zone.

The following functions are available to obtain the current date and/or time:

CURRENT_TIME and CURRENT_TIMESTAMP deliver values with time zone; LOCALTIME and LOCALTIMESTAMP deliver values without time zone.

CURRENT_TIME, CURRENT_TIMESTAMP, LOCALTIME, and LOCALTIMESTAMP can optionally be given a precision parameter, which causes the result to be rounded to that many fractional digits. Without a precision parameter, the result is given to the full available precision.

Note: Prior to PostgreSQL 7.2, the precision parameters were unimplemented, and the result was always given in integer seconds.

Some examples:

The function now() is the traditional PostgreSQL equivalent to CURRENT_TIMESTAMP.

There is also timeofday(), which for historical reasons returns a text string rather than a timestamp value:

It is important to realize that CURRENT_TIMESTAMP and related functions return the start time of the current transaction; their values do not change during the transaction. timeofday() returns the wall clock time and does advance during transactions.

Note: Many other database systems advance these values more frequently.

All the date/time data types also accept the special literal value now to specify the current date and time. Thus, the following three all return the same result:

Note: You do not want to use the third form when specifying a DEFAULT clause while creating a table. The system will convert now to a timestamp as soon as the constant is parsed, so that when the default value is needed, the time of the table creation would be used! The first two forms will not be evaluated until the default value is used, because they are function calls. Thus they will give the desired behavior of defaulting to the time of row insertion.

Page 16

This documentation is for an unsupported version of PostgreSQL.
You may want to view the same page for the current version, or one of the other supported versions listed above instead.

Table 4-14 shows the available functions for date/time value processing. The basic arithmetic operators (+, *, etc.) are also available. For formatting functions, refer to Section 4.6. You should be familiar with the background information on date/time data types (see Section 3.4).

Table 4-14. Date/Time Functions

EXTRACT (field FROM source)

The extract function retrieves sub-fields from date/time values, such as year or hour. source is a value expression that evaluates to type timestamp or interval. (Expressions of type date or time will be cast to timestamp and can therefore be used as well.) field is an identifier or string that selects what field to extract from the source value. The extract function returns values of type double precision. The following are valid values:

century

The year field divided by 100

Note that the result for the century field is simply the year field divided by 100, and not the conventional definition which puts most years in the 1900's in the twentieth century.

The day (of the month) field (1 - 31)

The year field divided by 10

The day of the week (0 - 6; Sunday is 0) (for timestamp values only)

The day of the year (1 - 365/366) (for timestamp values only)

For date and timestamp values, the number of seconds since 1970-01-01 00:00:00 (Result may be negative.); for interval values, the total number of seconds in the interval

The hour field (0 - 23)

The seconds field, including fractional parts, multiplied by 1 000 000. Note that this includes full seconds.

The year field divided by 1000

Note that the result for the millennium field is simply the year field divided by 1000, and not the conventional definition which puts years in the 1900's in the second millennium.

The seconds field, including fractional parts, multiplied by 1000. Note that this includes full seconds.

The minutes field (0 - 59)

For timestamp values, the number of the month within the year (1 - 12) ; for interval values the number of months, modulo 12 (0 - 11)

The quarter of the year (1 - 4) that the day is in (for timestamp values only)

The seconds field, including fractional parts (0 - 59[1])

From a timestamp value, calculate the number of the week of the year that the day is in. By definition (ISO 8601), the first week of a year contains January 4 of that year. (The ISO week starts on Monday.) In other words, the first Thursday of a year is in week 1 of that year.

The year field

The extract function is primarily intended for computational processing. For formatting date/time values for display, see Section 4.6.

The date_part function is modeled on the traditional Ingres equivalent to the SQL-function extract:

The function date_trunc is conceptually similar to the trunc function for numbers.

Valid values for field are:

The following functions are available to obtain the current date and/or time:

The function now() is the traditional Postgres equivalent to CURRENT_TIMESTAMP.

There is also timeofday(), which returns current time to higher precision than the CURRENT_TIMESTAMP family does:

timeofday() uses the operating system call gettimeofday(2), which may have resolution as good as microseconds (depending on your platform); the other functions rely on time(2) which is restricted to one-second resolution. For historical reasons, timeofday() returns its result as a text string rather than a timestamp value.

It is quite important to realize that CURRENT_TIMESTAMP and related functions all return the time as of the start of the current transaction; their values do not increment while a transaction is running. But timeofday() returns the actual current time.

All the date/time datatypes also accept the special literal value now to specify the current date and time. Thus, the following three all return the same result:

Note: You do not want to use the third form when specifying a DEFAULT value while creating a table. The system will convert now to a timestamp as soon as the constant is parsed, so that when the default value is needed, the time of the table creation would be used! The first two forms will not be evaluated until the default value is used, because they are function calls. Thus they will give the desired behavior of defaulting to the time of row insertion.