Table 9.26, “Date/Time Functions” shows the available
functions for date/time value processing, with details appearing in
the following subsections. Table 9.25, “Date/Time Operators” illustrates the behaviors of
the basic arithmetic operators (+
,
*
, etc.). For formatting functions, refer to
Section 9.8, “Data Type Formatting Functions”. You should be familiar with
the background information on date/time data types from Section 8.5, “Date/Time Types”.
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
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' |
- |
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 9.26. Date/Time Functions
Function | Return Type | Description | Example | Result |
---|---|---|---|---|
|
interval |
Subtract arguments, producing a “symbolic” result that uses years and months | age(timestamp '2001-04-10', timestamp '1957-06-13') |
43 years 9 mons 27 days |
|
interval |
Subtract from current_date
|
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, “Current Date/Time” | ||
|
date |
Current date; see Section 9.9.4, “Current Date/Time” | ||
|
time with time zone |
Current time of day; see Section 9.9.4, “Current Date/Time” | ||
|
timestamp with time zone |
Current date and time (start of current transaction); see Section 9.9.4, “Current Date/Time” | ||
|
double precision |
Get subfield (equivalent to extract );
see Section 9.9.1, “EXTRACT , date_part ”
|
date_part('hour', timestamp '2001-02-16 20:38:40') |
20 |
|
double precision |
Get subfield (equivalent to
extract ); see Section 9.9.1, “EXTRACT , date_part ”
|
date_part('month', interval '2 years 3 months') |
3 |
|
timestamp |
Truncate to specified precision; see also Section 9.9.2, “date_trunc ”
|
date_trunc('hour', timestamp '2001-02-16 20:38:40') |
2001-02-16 20:00:00 |
|
double precision |
Get subfield; see Section 9.9.1, “EXTRACT , date_part ”
|
extract(hour from timestamp '2001-02-16 20:38:40') |
20 |
|
double precision |
Get subfield; see Section 9.9.1, “EXTRACT , date_part ”
|
extract(month from interval '2 years 3 months') |
3 |
|
boolean |
Test for finite time stamp (not equal to 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 '30 days') |
1 month |
|
interval |
Adjust interval so 24-hour time periods are represented as days | justify_hours(interval '24 hours') |
1 day |
|
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, “Current Date/Time” | ||
|
timestamp |
Current date and time (start of current transaction); see Section 9.9.4, “Current Date/Time” | ||
|
timestamp with time zone |
Current date and time (start of current transaction); see Section 9.9.4, “Current Date/Time” | ||
|
timestamp with time zone |
Current date and time (start of current statement); see Section 9.9.4, “Current Date/Time” | ||
|
text |
Current date and time
(like clock_timestamp , but as a text string);
see Section 9.9.4, “Current Date/Time”
|
||
|
timestamp with time zone |
Current date and time (start of current transaction); see Section 9.9.4, “Current Date/Time” |
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.
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
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
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
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
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 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.
day
The day (of the month) field (1 - 31)
SELECT EXTRACT(DAY FROM TIMESTAMP '2001-02-16 20:38:40');
Result: 16
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 (0 - 6; Sunday is 0) (for
timestamp
values only)
SELECT EXTRACT(DOW FROM TIMESTAMP '2001-02-16 20:38:40');
Result: 5
Note that extract
's day of the week numbering is
different from that of the to_char
function.
doy
The day of the year (1 - 365/366) (for timestamp
values only)
SELECT EXTRACT(DOY FROM TIMESTAMP '2001-02-16 20:38:40');
Result: 47
epoch
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
SELECT EXTRACT(EPOCH FROM TIMESTAMP WITH TIME ZONE '2001-02-16 20:38:40-08'); Result:982384720
SELECT EXTRACT(EPOCH FROM INTERVAL '5 days 3 hours'); Result:442800
Here is how you can convert an epoch value back to a time stamp:
SELECT TIMESTAMP WITH TIME ZONE 'epoch' + 982384720 * INTERVAL '1 second';
hour
The hour field (0 - 23)
SELECT EXTRACT(HOUR FROM TIMESTAMP '2001-02-16 20:38:40');
Result: 20
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 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.
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 day is in (for
timestamp
values only)
SELECT EXTRACT(QUARTER FROM TIMESTAMP '2001-02-16 20:38:40');
Result: 1
second
The seconds field, including fractional parts (0 - 59[5])
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.
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 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.
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
The extract
function is primarily intended
for computational processing. For formatting date/time values for
display, see Section 9.8, “Data Type Formatting Functions”.
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
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
The AT TIME ZONE
construct allows conversions
of time stamps to different time zones. Table 9.27, “AT TIME ZONE
Variants” shows its
variants.
Table 9.27. 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 |
|
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., '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, “Time Zones”.
Examples (supposing that the local time zone is PST8PDT
):
SELECT TIMESTAMP '2001-02-16 20:38:40' AT TIME ZONE 'MST'; Result:2001-02-16 19:38:40-08
SELECT TIMESTAMP WITH TIME ZONE '2001-02-16 20:38:40-05' AT TIME ZONE 'MST'; Result:2001-02-16 18:38:40
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
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 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:
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 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() transaction_timestamp() statement_timestamp() clock_timestamp() timeofday()
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:
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 function is available to delay execution of the server process:
pg_sleep(seconds
)
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:
SELECT pg_sleep(1.5);
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.
Make sure that your session does not hold more locks than necessary
when calling pg_sleep
. Otherwise other sessions
might have to wait for your sleeping process, slowing down the entire
system.