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.
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
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 9.30. Date/Time Functions
Function | Return Type | Description | Example | Result |
---|---|---|---|---|
| interval | Subtract arguments, producing a “symbolic” result that uses years and months, rather than just days | age(timestamp '2001-04-10', timestamp '1957-06-13') | 43 years 9 mons 27 days |
| interval | Subtract from current_date (at midnight) | age(timestamp '1957-06-13') | 43 years 8 mons 3 days |
| timestamp with time zone | Current date and time (changes during statement execution); see Section 9.9.4 | ||
| date | Current date; see Section 9.9.4 | ||
| time with time zone | Current time of day; see Section 9.9.4 | ||
| timestamp with time zone | Current date and time (start of current transaction); see Section 9.9.4 | ||
| double precision | Get subfield (equivalent to extract );
see Section 9.9.1
| date_part('hour', timestamp '2001-02-16 20:38:40') | 20 |
| double precision | Get subfield (equivalent to
extract ); see Section 9.9.1
| date_part('month', interval '2 years 3 months') | 3 |
| timestamp | Truncate to specified precision; see also Section 9.9.2 | date_trunc('hour', timestamp '2001-02-16 20:38:40') | 2001-02-16 20:00:00 |
| interval | Truncate to specified precision; see also Section 9.9.2 | date_trunc('hour', interval '2 days 3 hours 40 minutes') | 2 days 03:00:00 |
| double precision | Get subfield; see Section 9.9.1 | extract(hour from timestamp '2001-02-16 20:38:40') | 20 |
| double precision | Get subfield; see Section 9.9.1 | extract(month from interval '2 years 3 months') | 3 |
| boolean | Test for finite date (not +/-infinity) | isfinite(date '2001-02-16') | true |
| boolean | Test for finite time stamp (not +/-infinity) | isfinite(timestamp '2001-02-16 21:28:30') | true |
| boolean | Test for finite interval | isfinite(interval '4 hours') | true |
| interval | Adjust interval so 30-day time periods are represented as months | justify_days(interval '35 days') | 1 mon 5 days |
| interval | Adjust interval so 24-hour time periods are represented as days | justify_hours(interval '27 hours') | 1 day 03:00:00 |
| 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 |
| time | Current time of day; see Section 9.9.4 | ||
| timestamp | Current date and time (start of current transaction); see Section 9.9.4 | ||
| date | Create date from year, month and day fields | make_date(2013, 7, 15) | 2013-07-15 |
| interval | Create interval from years, months, weeks, days, hours, minutes and seconds fields | make_interval(days => 10) | 10 days |
| time | Create time from hour, minute and seconds fields | make_time(8, 15, 23.5) | 08:15:23.5 |
| 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 |
| timestamp with time zone |
Create timestamp with time zone from year, month, day, hour, minute
and seconds fields; if timezone is not
specified, the current time zone is used
| make_timestamptz(2013, 7, 15, 8, 15, 23.5) | 2013-07-15 08:15:23.5+01 |
| timestamp with time zone | Current date and time (start of current transaction); see Section 9.9.4 | ||
| timestamp with time zone | Current date and time (start of current statement); see Section 9.9.4 | ||
| text | Current date and time
(like clock_timestamp , but as a text string);
see Section 9.9.4
| ||
| timestamp with time zone | Current date and time (start of current transaction); see Section 9.9.4 | ||
| timestamp with time zone | Convert Unix epoch (seconds since 1970-01-01 00:00:00+00) to timestamp | to_timestamp(1284352323) | 2010-09-13 04:32:03+00 |
In addition to these functions, the SQL OVERLAPS
operator is
supported:
(start1
,end1
) OVERLAPS (start2
,end2
) (start1
,length1
) OVERLAPS (start2
,length2
)
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.
SELECT (DATE '2001-02-16', DATE '2001-12-21') OVERLAPS (DATE '2001-10-30', DATE '2002-10-30'); Result:true
SELECT (DATE '2001-02-16', INTERVAL '100 days') OVERLAPS (DATE '2001-10-30', DATE '2002-10-30'); Result:false
SELECT (DATE '2001-10-29', DATE '2001-10-30') OVERLAPS (DATE '2001-10-30', DATE '2001-10-31'); Result:false
SELECT (DATE '2001-10-30', DATE '2001-10-30') OVERLAPS (DATE '2001-10-30', DATE '2001-10-31'); Result:true
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:
SELECT EXTRACT(EPOCH FROM timestamptz '2013-07-01 12:00:00') - EXTRACT(EPOCH FROM timestamptz '2013-03-01 12:00:00'); Result:10537200
SELECT (EXTRACT(EPOCH FROM timestamptz '2013-07-01 12:00:00') - EXTRACT(EPOCH FROM timestamptz '2013-03-01 12:00:00')) / 60 / 60 / 24; Result:121.958333333333
SELECT timestamptz '2013-07-01 12:00:00' - timestamptz '2013-03-01 12:00:00'; Result:121 days 23:00:00
SELECT age(timestamptz '2013-07-01 12:00:00', timestamptz '2013-03-01 12:00:00'); Result:4 mons
EXTRACT
, date_part
EXTRACT(field
FROMsource
)
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
SELECT EXTRACT(CENTURY FROM TIMESTAMP '2000-12-16 12:21:13'); Result:20
SELECT EXTRACT(CENTURY FROM TIMESTAMP '2001-02-16 20:38:40'); Result:21
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
SELECT EXTRACT(DAY FROM TIMESTAMP '2001-02-16 20:38:40'); Result:16
SELECT EXTRACT(DAY FROM INTERVAL '40 days 1 minute'); Result:40
decade
The year field divided by 10
SELECT EXTRACT(DECADE FROM TIMESTAMP '2001-02-16 20:38:40');
Result: 200
dow
The day of the week as Sunday (0
) to
Saturday (6
)
SELECT EXTRACT(DOW FROM TIMESTAMP '2001-02-16 20:38:40');
Result: 5
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)
SELECT EXTRACT(DOY FROM TIMESTAMP '2001-02-16 20:38:40');
Result: 47
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
SELECT EXTRACT(EPOCH FROM TIMESTAMP WITH TIME ZONE '2001-02-16 20:38:40.12-08'); Result:982384720.12
SELECT EXTRACT(EPOCH FROM INTERVAL '5 days 3 hours'); Result:442800
You can convert an epoch value back to a time stamp
with to_timestamp
:
SELECT to_timestamp(982384720.12);
Result: 2001-02-17 04:38:40.12+00
hour
The hour field (0 - 23)
SELECT EXTRACT(HOUR FROM TIMESTAMP '2001-02-16 20:38:40');
Result: 20
isodow
The day of the week as Monday (1
) to
Sunday (7
)
SELECT EXTRACT(ISODOW FROM TIMESTAMP '2001-02-18 20:38:40');
Result: 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)
SELECT EXTRACT(ISOYEAR FROM DATE '2006-01-01'); Result:2005
SELECT EXTRACT(ISOYEAR FROM DATE '2006-01-02'); Result:2006
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
SELECT EXTRACT(MICROSECONDS FROM TIME '17:12:28.5');
Result: 28500000
millennium
The millennium
SELECT EXTRACT(MILLENNIUM FROM TIMESTAMP '2001-02-16 20:38:40');
Result: 3
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.
SELECT EXTRACT(MILLISECONDS FROM TIME '17:12:28.5');
Result: 28500
minute
The minutes field (0 - 59)
SELECT EXTRACT(MINUTE FROM TIMESTAMP '2001-02-16 20:38:40');
Result: 38
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)
SELECT EXTRACT(MONTH FROM TIMESTAMP '2001-02-16 20:38:40'); Result:2
SELECT EXTRACT(MONTH FROM INTERVAL '2 years 3 months'); Result:3
SELECT EXTRACT(MONTH FROM INTERVAL '2 years 13 months'); Result:1
quarter
The quarter of the year (1 - 4) that the date is in
SELECT EXTRACT(QUARTER FROM TIMESTAMP '2001-02-16 20:38:40');
Result: 1
second
The seconds field, including fractional parts (0 - 59[7])
SELECT EXTRACT(SECOND FROM TIMESTAMP '2001-02-16 20:38:40'); Result:40
SELECT EXTRACT(SECOND FROM TIME '17:12:28.5'); Result:28.5
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 does not use UTC because leap seconds are not handled.)
timezone_hour
The hour component of the time zone offset
timezone_minute
The minute component of the time zone offset
week
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.
SELECT EXTRACT(WEEK FROM TIMESTAMP '2001-02-16 20:38:40');
Result: 7
year
The year field. Keep in mind there is no 0 AD
, so subtracting
BC
years from AD
years should be done with care.
SELECT EXTRACT(YEAR FROM TIMESTAMP '2001-02-16 20:38:40');
Result: 2001
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
:
date_part('field
',source
)
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
.
SELECT date_part('day', TIMESTAMP '2001-02-16 20:38:40'); Result:16
SELECT date_part('hour', INTERVAL '4 hours 3 minutes'); Result:4
date_trunc
The function date_trunc
is conceptually
similar to the trunc
function for numbers.
date_trunc('field
',source
)
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:
SELECT date_trunc('hour', TIMESTAMP '2001-02-16 20:38:40'); Result:2001-02-16 20:00:00
SELECT date_trunc('year', TIMESTAMP '2001-02-16 20:38:40'); Result:2001-01-01 00:00:00
AT TIME ZONE
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
Expression | Return Type | Description |
---|---|---|
| timestamp with time zone | Treat given time stamp without time zone as located in the specified time 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 | Convert given time with time zone to the new time zone |
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
):
SELECT TIMESTAMP '2001-02-16 20:38:40' AT TIME ZONE 'America/Denver'; Result:2001-02-16 19:38:40-08
SELECT TIMESTAMP WITH TIME ZONE '2001-02-16 20:38:40-05' AT TIME ZONE 'America/Denver'; Result:2001-02-16 18:38:40
SELECT TIMESTAMP '2001-02-16 20:38:40-05' AT TIME ZONE 'Asia/Tokyo' AT TIME ZONE 'America/Chicago'; Result:2001-02-16 05:38:40
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
is equivalent to the SQL-conforming construct
timezone
(zone
,
timestamp
)
.
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_DATE CURRENT_TIME CURRENT_TIMESTAMP CURRENT_TIME(precision
) CURRENT_TIMESTAMP(precision
) LOCALTIME LOCALTIMESTAMP LOCALTIME(precision
) LOCALTIMESTAMP(precision
)
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:
SELECT CURRENT_TIME; Result:14:39:53.662522-05
SELECT CURRENT_DATE; Result:2001-12-23
SELECT CURRENT_TIMESTAMP; Result:2001-12-23 14:39:53.662522-05
SELECT CURRENT_TIMESTAMP(2); Result:2001-12-23 14:39:53.66-05
SELECT LOCALTIMESTAMP; Result:2001-12-23 14:39:53.662522
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.
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() statement_timestamp() clock_timestamp() timeofday() now()
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:
SELECT CURRENT_TIMESTAMP; SELECT now(); SELECT TIMESTAMP 'now'; -- incorrect for use with DEFAULT
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(seconds
) pg_sleep_for(interval
) pg_sleep_until(timestamp with time zone
)
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:
SELECT pg_sleep(1.5); SELECT pg_sleep_for('5 minutes'); SELECT pg_sleep_until('tomorrow 03:00');
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.
Make sure that your session does not hold more locks than necessary
when calling pg_sleep
or its variants. Otherwise
other sessions might have to wait for your sleeping process, slowing down
the entire system.