Table 9.33 shows the available
functions for date/time value processing, with details appearing in
the following subsections. Table 9.32 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 9.32. Date/Time Operators
Operator Description Example(s) |
|---|
Add a number of days to a date
|
Add an interval to a date
|
Add a time-of-day to a date
|
Add intervals
|
Add an interval to a timestamp
|
Add an interval to a time
|
Negate an interval
|
Subtract dates, producing the number of days elapsed
|
Subtract a number of days from a date
|
Subtract an interval from a date
|
Subtract times
|
Subtract an interval from a time
|
Subtract an interval from a timestamp
|
Subtract intervals
|
Subtract timestamps (converting 24-hour intervals into days,
similarly to
|
Multiply an interval by a scalar
|
Divide an interval by a scalar
|
Table 9.33. Date/Time Functions
Function Description Example(s) |
|---|
Subtract arguments, producing a “symbolic” result that uses years and months, rather than just days
|
Subtract argument from
|
Current date and time (changes during statement execution); see Section 9.9.5
|
|
Current date; see Section 9.9.5
|
Current time of day; see Section 9.9.5
|
Current time of day, with limited precision; see Section 9.9.5
|
Current date and time (start of current transaction); see Section 9.9.5
|
Current date and time (start of current transaction), with limited precision; see Section 9.9.5
|
Add an
|
Bin input into specified interval aligned with specified origin; see Section 9.9.3
|
Get timestamp subfield (equivalent to
|
Get interval subfield (equivalent to
|
Subtract an
|
Truncate to specified precision; see Section 9.9.2
|
Truncate to specified precision in the specified time zone; see Section 9.9.2
|
Truncate to specified precision; see Section 9.9.2
|
Get timestamp subfield; see Section 9.9.1
|
Get interval subfield; see Section 9.9.1
|
|
Test for finite date (not +/-infinity)
|
Test for finite timestamp (not +/-infinity)
|
Test for finite interval (not +/-infinity)
|
Adjust interval, converting 30-day time periods to months
|
Adjust interval, converting 24-hour time periods to days
|
Adjust interval using
|
|
Current time of day; see Section 9.9.5
|
Current time of day, with limited precision; see Section 9.9.5
|
|
Current date and time (start of current transaction); see Section 9.9.5
|
Current date and time (start of current transaction), with limited precision; see Section 9.9.5
|
Create date from year, month and day fields (negative years signify BC)
|
Create interval from years, months, weeks, days, hours, minutes and seconds fields, each of which can default to zero
|
Create time from hour, minute and seconds fields
|
Create timestamp from year, month, day, hour, minute and seconds fields (negative years signify BC)
|
Create timestamp with time zone from year, month, day, hour, minute
and seconds fields (negative years signify BC).
If
|
Current date and time (start of current transaction); see Section 9.9.5
|
Current date and time (start of current statement); see Section 9.9.5
|
|
Current date and time
(like
|
Current date and time (start of current transaction); see Section 9.9.5
|
Convert Unix epoch (seconds since 1970-01-01 00:00:00+00) to timestamp with time zone
|
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
or timestamp with time zone value, the months, days, and
microseconds fields of the interval value are handled in turn.
First, a nonzero months field advances or decrements the date of the
timestamp by the indicated number of months, keeping the day of month the
same unless it would be past the end of the new month, in which case the
last day of that month is used. (For example, March 31 plus 1 month
becomes April 30, but March 31 plus 2 months becomes May 31.)
Then the days field advances or decrements the date of the timestamp by
the indicated number of days. In both these steps the local time of day
is kept the same. Finally, if there is a nonzero microseconds field, it
is added or subtracted literally.
When doing arithmetic on a timestamp with time zone value in
a time zone that recognizes DST, this means that adding or subtracting
(say) interval '1 day' does not necessarily have the
same result as adding or subtracting interval '24
hours'.
For example, with the session time zone set
to America/Denver:
SELECT timestamp with time zone '2005-04-02 12:00:00-07' + interval '1 day'; Result:2005-04-03 12:00:00-06SELECT timestamp with time zone '2005-04-02 12:00:00-07' + interval '24 hours'; Result:2005-04-03 13:00:00-06
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:
SELECT EXTRACT(EPOCH FROM timestamptz '2013-07-01 12:00:00') -
EXTRACT(EPOCH FROM timestamptz '2013-03-01 12:00:00');
Result: 10537200.000000
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: 121.9583333333333333
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(fieldFROMsource)
The extract function retrieves subfields
such as year or hour from date/time values.
source must be a value expression of
type timestamp, date, time,
or interval. (Timestamps and times can be with or
without time zone.)
field is an identifier or
string that selects what field to extract from the source value.
Not all fields are valid for every input data type; for example, fields
smaller than a day cannot be extracted from a date, while
fields of a day or more cannot be extracted from a time.
The extract function returns values of type
numeric.
The following are valid field names:
century
The century; for interval values, the year field
divided by 100
SELECT EXTRACT(CENTURY FROM TIMESTAMP '2000-12-16 12:21:13'); Result:20SELECT EXTRACT(CENTURY FROM TIMESTAMP '2001-02-16 20:38:40'); Result:21SELECT EXTRACT(CENTURY FROM DATE '0001-01-01 AD'); Result:1SELECT EXTRACT(CENTURY FROM DATE '0001-12-31 BC'); Result:-1SELECT EXTRACT(CENTURY FROM INTERVAL '2001 years'); Result:20
day
The day of the month (1–31); for interval
values, the number of days
SELECT EXTRACT(DAY FROM TIMESTAMP '2001-02-16 20:38:40'); Result:16SELECT EXTRACT(DAY FROM INTERVAL '40 days 1 minute'); Result:40
decadeThe 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.
doyThe 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 (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
SELECT EXTRACT(EPOCH FROM TIMESTAMP WITH TIME ZONE '2001-02-16 20:38:40.12-08'); Result:982384720.120000SELECT EXTRACT(EPOCH FROM TIMESTAMP '2001-02-16 20:38:40.12'); Result:982355920.120000SELECT EXTRACT(EPOCH FROM INTERVAL '5 days 3 hours'); Result:442800.000000
You can convert an epoch value back to a timestamp with time zone
with to_timestamp:
SELECT to_timestamp(982384720.12);
Result: 2001-02-17 04:38:40.12+00
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.
hourThe hour field (0–23 in timestamps, unrestricted in intervals)
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.
isoyearThe ISO 8601 week-numbering year that the date falls in
SELECT EXTRACT(ISOYEAR FROM DATE '2006-01-01'); Result:2005SELECT 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.
julianThe Julian Date corresponding to the date or timestamp. Timestamps that are not local midnight result in a fractional value. See Section B.7 for more information.
SELECT EXTRACT(JULIAN FROM DATE '2006-01-01'); Result:2453737SELECT EXTRACT(JULIAN FROM TIMESTAMP '2006-01-01 12:00'); Result:2453737.50000000000000000000
microsecondsThe 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; for interval values, the year field
divided by 1000
SELECT EXTRACT(MILLENNIUM FROM TIMESTAMP '2001-02-16 20:38:40'); Result:3SELECT EXTRACT(MILLENNIUM FROM INTERVAL '2001 years'); Result:2
Years in the 1900s are in the second millennium. The third millennium started January 1, 2001.
millisecondsThe seconds field, including fractional parts, multiplied by 1000. Note that this includes full seconds.
SELECT EXTRACT(MILLISECONDS FROM TIME '17:12:28.5');
Result: 28500.000
minuteThe minutes field (0–59)
SELECT EXTRACT(MINUTE FROM TIMESTAMP '2001-02-16 20:38:40');
Result: 38
month
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:2SELECT EXTRACT(MONTH FROM INTERVAL '2 years 3 months'); Result:3SELECT EXTRACT(MONTH FROM INTERVAL '2 years 13 months'); Result:1
quarterThe quarter of the year (1–4) that the date is in
SELECT EXTRACT(QUARTER FROM TIMESTAMP '2001-02-16 20:38:40');
Result: 1
secondThe seconds field, including any fractional seconds
SELECT EXTRACT(SECOND FROM TIMESTAMP '2001-02-16 20:38:40'); Result:40.000000SELECT EXTRACT(SECOND FROM TIME '17:12:28.5'); Result:28.500000
timezoneThe 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_hourThe hour component of the time zone offset
timezone_minuteThe minute component of the time zone offset
weekThe 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 processing an interval value,
the extract function produces field values that
match the interpretation used by the interval output function. This
can produce surprising results if one starts with a non-normalized
interval representation, for example:
SELECT INTERVAL '80 minutes'; Result:01:20:00SELECT EXTRACT(MINUTES FROM INTERVAL '80 minutes'); Result:20
When the input value is +/-Infinity, extract returns
+/-Infinity for monotonically-increasing fields (epoch,
julian, year, isoyear,
decade, century, and millennium
for timestamp inputs; epoch, hour,
day, year, decade,
century, and millennium for
interval inputs).
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.
For historical reasons, the date_part function
returns values of type double precision. This can result in
a loss of precision in certain uses. Using extract
is recommended instead.
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[,time_zone])
source is a value expression of type
timestamp, timestamp with time zone,
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 likewise of type
timestamp, timestamp with time zone,
or interval,
and it has 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 |
When the input value is of type timestamp with time zone,
the truncation is performed with respect to a particular time zone;
for example, truncation to day produces a value that
is midnight in that zone. By default, truncation is done with respect
to the current TimeZone setting, but the
optional time_zone argument can be provided
to specify a different time zone. The time zone name can be specified
in any of the ways described in Section 8.5.3.
A time zone cannot be specified when processing timestamp without
time zone or interval inputs. These are always
taken at face value.
Examples (assuming the local time zone is America/New_York):
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
SELECT date_trunc('day', TIMESTAMP WITH TIME ZONE '2001-02-16 20:38:40+00');
Result: 2001-02-16 00:00:00-05
SELECT date_trunc('day', TIMESTAMP WITH TIME ZONE '2001-02-16 20:38:40+00', 'Australia/Sydney');
Result: 2001-02-16 08:00:00-05
SELECT date_trunc('hour', INTERVAL '3 days 02:47:33');
Result: 3 days 02:00:00
date_bin #
The function date_bin “bins” the input
timestamp into the specified interval (the stride)
aligned with a specified origin.
date_bin(stride,source,origin)
source is a value expression of type
timestamp or timestamp with time zone. (Values
of type date are cast automatically to
timestamp.) stride is a value
expression of type interval. The return value is likewise
of type timestamp or timestamp with time zone,
and it marks the beginning of the bin into which the
source is placed.
Examples:
SELECT date_bin('15 minutes', TIMESTAMP '2020-02-11 15:44:17', TIMESTAMP '2001-01-01');
Result: 2020-02-11 15:30:00
SELECT date_bin('15 minutes', TIMESTAMP '2020-02-11 15:44:17', TIMESTAMP '2001-01-01 00:02:30');
Result: 2020-02-11 15:32:30
In the case of full units (1 minute, 1 hour, etc.), it gives the same result as
the analogous date_trunc call, but the difference is
that date_bin can truncate to an arbitrary interval.
The stride interval must be greater than zero and
cannot contain units of month or larger.
AT TIME ZONE and AT LOCAL #
The AT TIME ZONE operator converts time
stamp without time zone to/from
time stamp with time zone, and
time with time zone values to different time
zones. Table 9.34 shows its
variants.
Table 9.34. AT TIME ZONE and AT LOCAL Variants
Operator Description Example(s) |
|---|
Converts given time stamp without time zone to time stamp with time zone, assuming the given value is in the named time zone.
|
Converts given time stamp without time zone to
time stamp with the session's
|
Converts given time stamp with time zone to time stamp without time zone, as the time would appear in that zone.
|
Converts given time stamp with time zone to
time stamp without time zone, as the time would
appear with the session's
|
Converts given time with time zone to a new time zone. Since no date is supplied, this uses the currently active UTC offset for the named destination zone.
|
Converts given time with time zone to a new time
zone. Since no date is supplied, this uses the currently active UTC
offset for the session's
Assuming the session's
|
In these expressions, the desired time zone zone can be
specified either as a text value (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.
The interval case is only useful for zones that have fixed offsets from
UTC, so it is not very common in practice.
The syntax AT LOCAL may be used as shorthand for
AT TIME ZONE , where
locallocal is the session's
TimeZone value.
Examples (assuming the current TimeZone setting
is America/Los_Angeles):
SELECT TIMESTAMP '2001-02-16 20:38:40' AT TIME ZONE 'America/Denver'; Result:2001-02-16 19:38:40-08SELECT TIMESTAMP WITH TIME ZONE '2001-02-16 20:38:40-05' AT TIME ZONE 'America/Denver'; Result:2001-02-16 18:38:40SELECT TIMESTAMP '2001-02-16 20:38:40' AT TIME ZONE 'Asia/Tokyo' AT TIME ZONE 'America/Chicago'; Result:2001-02-16 05:38:40SELECT TIMESTAMP WITH TIME ZONE '2001-02-16 20:38:40-05' AT LOCAL; Result:2001-02-16 17:38:40SELECT TIME WITH TIME ZONE '20:38:40-05' AT LOCAL; Result:17:38:40
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. The fourth example shifts the time stamp
with time zone value to the time zone currently specified by the
TimeZone setting and returns the value without a
time zone.
The fifth example is a cautionary tale. Due to the fact that there is no
date associated with the input value, the conversion is made using the
current date of the session. Therefore, this static example may show a wrong
result depending on the time of the year it is viewed because
'America/Los_Angeles' observes Daylight Savings Time.
The function timezone(zone,
timestamp)timestamp AT TIME ZONE
zone
The function timezone(zone,
time)time AT TIME ZONE
zone
The function timezone(timestamp)timestamp
AT LOCAL
The function timezone(time)time
AT LOCAL
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:14:39:53.662522-05SELECT CURRENT_DATE; Result:2019-12-23SELECT CURRENT_TIMESTAMP; Result:2019-12-23 14:39:53.662522-05SELECT CURRENT_TIMESTAMP(2); Result:2019-12-23 14:39:53.66-05SELECT LOCALTIMESTAMP; Result:2019-12-23 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'; -- but see tip below
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 (double precision) pg_sleep_for (interval) pg_sleep_until (timestamp with time zone)
pg_sleep makes the current session's process
sleep until the given number of seconds have
elapsed. Fractional-second delays can be specified.
pg_sleep_for is a convenience function to
allow the sleep time to be 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.