9.9 日期時間函式及運算子
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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 . You should be familiar with the background information on date/time data types from .
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
Operator
Example
Result
+
date '2001-09-28' + integer '7'
date '2001-10-05'
+
date '2001-09-28' + interval '1 hour'
timestamp '2001-09-28 01:00:00'
+
date '2001-09-28' + time '03:00'
timestamp '2001-09-28 03:00:00'
+
interval '1 day' + interval '1 hour'
interval '1 day 01:00:00'
+
timestamp '2001-09-28 01:00' + interval '23 hours'
timestamp '2001-09-29 00:00:00'
+
time '01:00' + interval '3 hours'
time '04:00:00'
-
- interval '23 hours'
interval '-23:00:00'
-
date '2001-10-01' - date '2001-09-28'
integer '3' (days)
-
date '2001-10-01' - integer '7'
date '2001-09-24'
-
date '2001-09-28' - interval '1 hour'
timestamp '2001-09-27 23:00:00'
-
time '05:00' - time '03:00'
interval '02:00:00'
-
time '05:00' - interval '2 hours'
time '03:00:00'
-
timestamp '2001-09-28 23:00' - interval '23 hours'
timestamp '2001-09-28 00:00:00'
-
interval '1 day' - interval '1 hour'
interval '1 day -01:00:00'
-
timestamp '2001-09-29 03:00' - timestamp '2001-09-27 12:00'
interval '1 day 15:00:00'
*
900 * interval '1 second'
interval '00:15:00'
*
21 * interval '1 day'
interval '21 days'
*
double precision '3.5' * interval '1 hour'
interval '03:30:00'
/
interval '1 hour' / double precision '1.5'
interval '00:40:00'
Table 9-28. Date/Time Functions
Function
Return Type
Description
Example
Result
age(timestamp, timestamp)
interval
參數間相減，產生一個使用年和月的帶有「符號」的結果，而不僅僅是幾天
age(timestamp '2001-04-10', timestamp '1957-06-13')
43 years 9 mons 27 days
age(timestamp)
interval
用 current_date 減去該日期（以午夜為準）
age(timestamp '1957-06-13')
43 years 8 mons 3 days
clock_timestamp()
timestamp with time zone
current_date
date
current_time
time with time zone
current_timestamp
timestamp with time zone
date_part(text, timestamp)
double precision
date_part('hour', timestamp '2001-02-16 20:38:40')
20
date_part(text, interval)
double precision
date_part('month', interval '2 years 3 months')
3
date_trunc(text, timestamp)
timestamp
date_trunc('hour', timestamp '2001-02-16 20:38:40')
2001-02-16 20:00:00
date_trunc(text, interval)
interval
date_trunc('hour', interval '2 days 3 hours 40 minutes')
2 days 03:00:00
extract(field from timestamp)
double precision
extract(hour from timestamp '2001-02-16 20:38:40')
20
extract(field from interval)
double precision
extract(month from interval '2 years 3 months')
3
isfinite(date)
boolean
Test for finite date (not +/-infinity)
isfinite(date '2001-02-16')
true
isfinite(timestamp)
boolean
Test for finite time stamp (not +/-infinity)
isfinite(timestamp '2001-02-16 21:28:30')
true
isfinite(interval)
boolean
Test for finite interval
isfinite(interval '4 hours')
true
justify_days(interval)
interval
Adjust interval so 30-day time periods are represented as months
justify_days(interval '35 days')
1 mon 5 days
justify_hours(interval)
interval
Adjust interval so 24-hour time periods are represented as days
justify_hours(interval '27 hours')
1 day 03:00:00
justify_interval(interval)
interval
Adjust interval using justify_days and justify_hours, with additional sign adjustments
justify_interval(interval '1 mon -1 hour')
29 days 23:00:00
localtime
time
localtimestamp
timestamp
make_date(year int, month int, day int)
date
Create date from year, month and day fields
make_date(2013, 7, 15)
2013-07-15
make_interval(years int DEFAULT 0, months int DEFAULT 0, weeksint DEFAULT 0, days int DEFAULT 0, hours int DEFAULT 0, mins intDEFAULT 0, secs double precision DEFAULT 0.0)
interval
Create interval from years, months, weeks, days, hours, minutes and seconds fields
make_interval(days := 10)
10 days
make_time(hour int, min int, sec double precision)
time
Create time from hour, minute and seconds fields
make_time(8, 15, 23.5)
08:15:23.5
make_timestamp(year int, month int, day int, hour int, min int,sec double precision)
timestamp
Create timestamp from year, month, day, hour, minute and seconds fields
make_timestamp(2013, 7, 15, 8, 15, 23.5)
2013-07-15 08:15:23.5
make_timestamptz(year int, month int, day int, hour int, min int,sec double precision, [ timezone text ])
timestamp with time zone
Create timestamp with time zone from year, month, day, hour, minute and seconds fields. When timezone is not specified, then current time zone is used.
make_timestamptz(2013, 7, 15, 8, 15, 23.5)
2013-07-15 08:15:23.5+01
now()
timestamp with time zone
statement_timestamp()
timestamp with time zone
timeofday()
text
transaction_timestamp()
timestamp with time zone
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, date_partThe 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 dateare 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.day
For timestamp values, the day (of the month) field (1 - 31) ; for interval values, the number of days
decade
The year field divided by 10
dow
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.doy
The day of the year (1 - 365/366)
epoch
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.)hour
The hour field (0 - 23)
isodow
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.isoyear
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.microseconds
The seconds field, including fractional parts, multiplied by 1 000 000; note that this includes full seconds
millennium
The millennium
Years in the 1900s are in the second millennium. The third millennium started January 1, 2001.milliseconds
The seconds field, including fractional parts, multiplied by 1000. Note that this includes full seconds.
minute
The minutes field (0 - 59)
month
For timestamp values, the number of the month within the year (1 - 12) ; for interval values, the number of months, modulo 12 (0 - 11)
quarter
The quarter of the year (1 - 4) that the date is in
second
timezone
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.)timezone_hour
The hour component of the time zone offsettimezone_minute
The minute component of the time zone offsetweek
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.
year
The year field. Keep in mind there is no 0 AD, so subtracting BC years from AD years should be done with care.
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.
date_truncThe 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:
microseconds
milliseconds
second
minute
hour
day
week
month
quarter
year
decade
century
millennium
Examples:
Table 9-29. AT TIME ZONE Variants
Expression
Return Type
Description
timestamp without time zone AT TIME ZONE zone
timestamp with time zone
Treat given time stamp without time zone as located in the specified time zone
timestamp with time zone AT TIME ZONE zone
timestamp without time zone
Convert given time stamp with time zone to the new time zone, with no time zone designation
time with time zone AT TIME ZONE zone
time with time zone
Convert given time with time zone to the new time zone
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 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_untilis not guaranteed to wake up exactly at the specified time, but it will not wake up any earlier.
Warning
Make sure that your session does not hold more locks than necessary when calling pg_sleepor its variants. Otherwise other sessions might have to wait for your sleeping process, slowing down the entire system.
60 if leap seconds are implemented by the operating system
Current date and time (changes during statement execution); see
目前日期；詳見第
Current time of day; see
Current date and time (start of current transaction); see
Get subfield (equivalent to extract); see
Get subfield (equivalent to extract); see
Truncate to specified precision; see also
Truncate to specified precision; see also
Get subfield; see
Get subfield; see
Current time of day; see
Current date and time (start of current transaction); see
Current date and time (start of current transaction); see
Current date and time (start of current statement); see
Current date and time (like clock_timestamp, but as a textstring); see
Current date and time (start of current transaction); see
The seconds field, including fractional parts (0 - 59)
The extract function is primarily intended for computational processing. For formatting date/time values for display, see .
The AT TIME ZONE construct allows conversions of time stamps to different time zones. shows its 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 .