Date/Time Functions and Operators (Postgres)

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--------------------

# 9.9. Date/Time Functions and Operators #

[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 taketime ortimestamp inputs actually come in two variants: one that takestime with time zone ortimestamp with time zone, and one that takestime without time zone ortimestamp 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>      
 <thead> 
  <tr> 
   <th>Operator</th> 
   <th>Example</th> 
   <th>Result</th> 
  </tr> 
 </thead> 
 <tbody> 
  <tr> 
   <td>+</td> 
   <td>date '2001-09-28' + integer '7'</td> 
   <td>date '2001-10-05'</td> 
  </tr> 
  <tr> 
   <td>+</td> 
   <td>date '2001-09-28' + interval '1 hour'</td> 
   <td>timestamp '2001-09-28 01:00:00'</td> 
  </tr> 
  <tr> 
   <td>+</td> 
   <td>date '2001-09-28' + time '03:00'</td> 
   <td>timestamp '2001-09-28 03:00:00'</td> 
  </tr> 
  <tr> 
   <td>+</td> 
   <td>interval '1 day' + interval '1 hour'</td> 
   <td>interval '1 day 01:00:00'</td> 
  </tr> 
  <tr> 
   <td>+</td> 
   <td>timestamp '2001-09-28 01:00' + interval '23 hours'</td> 
   <td>timestamp '2001-09-29 00:00:00'</td> 
  </tr> 
  <tr> 
   <td>+</td> 
   <td>time '01:00' + interval '3 hours'</td> 
   <td>time '04:00:00'</td> 
  </tr> 
  <tr> 
   <td>-</td> 
   <td>- interval '23 hours'</td> 
   <td>interval '-23:00:00'</td> 
  </tr> 
  <tr> 
   <td>-</td> 
   <td>date '2001-10-01' - date '2001-09-28'</td> 
   <td>integer '3'<span>&nbsp;(days)</span></td> 
  </tr> 
  <tr> 
   <td>-</td> 
   <td>date '2001-10-01' - integer '7'</td> 
   <td>date '2001-09-24'</td> 
  </tr> 
  <tr> 
   <td>-</td> 
   <td>date '2001-09-28' - interval '1 hour'</td> 
   <td>timestamp '2001-09-27 23:00:00'</td> 
  </tr> 
  <tr> 
   <td>-</td> 
   <td>time '05:00' - time '03:00'</td> 
   <td>interval '02:00:00'</td> 
  </tr> 
  <tr> 
   <td>-</td> 
   <td>time '05:00' - interval '2 hours'</td> 
   <td>time '03:00:00'</td> 
  </tr> 
  <tr> 
   <td>-</td> 
   <td>timestamp '2001-09-28 23:00' - interval '23 hours'</td> 
   <td>timestamp '2001-09-28 00:00:00'</td> 
  </tr> 
  <tr> 
   <td>-</td> 
   <td>interval '1 day' - interval '1 hour'</td> 
   <td>interval '1 day -01:00:00'</td> 
  </tr> 
  <tr> 
   <td>-</td> 
   <td>timestamp '2001-09-29 03:00' - timestamp '2001-09-27 12:00'</td> 
   <td>interval '1 day 15:00:00'</td> 
  </tr> 
  <tr> 
   <td>*</td> 
   <td>900 * interval '1 second'</td> 
   <td>interval '00:15:00'</td> 
  </tr> 
  <tr> 
   <td>*</td> 
   <td>21 * interval '1 day'</td> 
   <td>interval '21 days'</td> 
  </tr> 
  <tr> 
   <td>*</td> 
   <td>double precision '3.5' * interval '1 hour'</td> 
   <td>interval '03:30:00'</td> 
  </tr> 
  <tr> 
   <td>/</td> 
   <td>interval '1 hour' / double precision '1.5'</td> 
   <td>interval '00:40:00'</td> 
  </tr> 
 </tbody> 
</table>

Table 9-28. Date/Time Functions

<table>        
 <thead> 
  <tr> 
   <th>Function</th> 
   <th>Return Type</th> 
   <th>Description</th> 
   <th>Example</th> 
   <th>Result</th> 
  </tr> 
 </thead> 
 <tbody> 
  <tr> 
   <td><code>age(timestamp,<span>&nbsp;timestamp)</span></code></td> 
   <td>interval</td> 
   <td>Subtract arguments, producing a<span>&nbsp;<span>"symbolic"<span>&nbsp;result that uses years and months</span></span></span></td> 
   <td>age(timestamp '2001-04-10', timestamp '1957-06-13')</td> 
   <td>43 years 9 mons 27 days</td> 
  </tr> 
  <tr> 
   <td><code>age(timestamp)</code></td> 
   <td>interval</td> 
   <td>Subtract from<span>&nbsp;<code>current_date</code><span>&nbsp;(at midnight)</span></span></td> 
   <td>age(timestamp '1957-06-13')</td> 
   <td>43 years 8 mons 3 days</td> 
  </tr> 
  <tr> 
   <td><code>clock_timestamp()</code></td> 
   <td>timestamp with time zone</td> 
   <td>Current date and time (changes during statement execution); see<span>&nbsp;<a href="http://www.postgresql.org/docs/9.1/static/functions-datetime.html#FUNCTIONS-DATETIME-CURRENT" rel="nofollow">Section 9.9.4</a></span></td> 
   <td>&nbsp;</td> 
   <td>&nbsp;</td> 
  </tr> 
  <tr> 
   <td><code>current_date</code></td> 
   <td>date</td> 
   <td>Current date; see<span>&nbsp;<a href="http://www.postgresql.org/docs/9.1/static/functions-datetime.html#FUNCTIONS-DATETIME-CURRENT" rel="nofollow">Section 9.9.4</a></span></td> 
   <td>&nbsp;</td> 
   <td>&nbsp;</td> 
  </tr> 
  <tr> 
   <td><code>current_time</code></td> 
   <td>time with time zone</td> 
   <td>Current time of day; see<span>&nbsp;<a href="http://www.postgresql.org/docs/9.1/static/functions-datetime.html#FUNCTIONS-DATETIME-CURRENT" rel="nofollow">Section 9.9.4</a></span></td> 
   <td>&nbsp;</td> 
   <td>&nbsp;</td> 
  </tr> 
  <tr> 
   <td><code>current_timestamp</code></td> 
   <td>timestamp with time zone</td> 
   <td>Current date and time (start of current transaction); see<span>&nbsp;<a href="http://www.postgresql.org/docs/9.1/static/functions-datetime.html#FUNCTIONS-DATETIME-CURRENT" rel="nofollow">Section 9.9.4</a></span></td> 
   <td>&nbsp;</td> 
   <td>&nbsp;</td> 
  </tr> 
  <tr> 
   <td><code>date_part(text,<span>&nbsp;timestamp)</span></code></td> 
   <td>double precision</td> 
   <td>Get subfield (equivalent to<span>&nbsp;<code>extract</code>); see<span>&nbsp;<a href="http://www.postgresql.org/docs/9.1/static/functions-datetime.html#FUNCTIONS-DATETIME-EXTRACT" rel="nofollow">Section 9.9.1</a></span></span></td> 
   <td>date_part('hour', timestamp '2001-02-16 20:38:40')</td> 
   <td>20</td> 
  </tr> 
  <tr> 
   <td><code>date_part(text,<span>&nbsp;interval)</span></code></td> 
   <td>double precision</td> 
   <td>Get subfield (equivalent to<span>&nbsp;<code>extract</code>); see<span>&nbsp;<a href="http://www.postgresql.org/docs/9.1/static/functions-datetime.html#FUNCTIONS-DATETIME-EXTRACT" rel="nofollow">Section 9.9.1</a></span></span></td> 
   <td>date_part('month', interval '2 years 3 months')</td> 
   <td>3</td> 
  </tr> 
  <tr> 
   <td><code>date_trunc(text,timestamp)</code></td> 
   <td>timestamp</td> 
   <td>Truncate to specified precision; see also<span>&nbsp;<a href="http://www.postgresql.org/docs/9.1/static/functions-datetime.html#FUNCTIONS-DATETIME-TRUNC" rel="nofollow">Section 9.9.2</a></span></td> 
   <td>date_trunc('hour', timestamp '2001-02-16 20:38:40')</td> 
   <td>2001-02-16 20:00:00</td> 
  </tr> 
  <tr> 
   <td><code>extract</code>(field<span>&nbsp;fromtimestamp)</span></td> 
   <td>double precision</td> 
   <td>Get subfield; see<span>&nbsp;<a href="http://www.postgresql.org/docs/9.1/static/functions-datetime.html#FUNCTIONS-DATETIME-EXTRACT" rel="nofollow">Section 9.9.1</a></span></td> 
   <td>extract(hour from timestamp '2001-02-16 20:38:40')</td> 
   <td>20</td> 
  </tr> 
  <tr> 
   <td><code>extract</code>(field<span>&nbsp;frominterval)</span></td> 
   <td>double precision</td> 
   <td>Get subfield; see<span>&nbsp;<a href="http://www.postgresql.org/docs/9.1/static/functions-datetime.html#FUNCTIONS-DATETIME-EXTRACT" rel="nofollow">Section 9.9.1</a></span></td> 
   <td>extract(month from interval '2 years 3 months')</td> 
   <td>3</td> 
  </tr> 
  <tr> 
   <td><code>isfinite(date)</code></td> 
   <td>boolean</td> 
   <td>Test for finite date (not +/-infinity)</td> 
   <td>isfinite(date '2001-02-16')</td> 
   <td>true</td> 
  </tr> 
  <tr> 
   <td><code>isfinite(timestamp)</code></td> 
   <td>boolean</td> 
   <td>Test for finite time stamp (not +/-infinity)</td> 
   <td>isfinite(timestamp '2001-02-16 21:28:30')</td> 
   <td>true</td> 
  </tr> 
  <tr> 
   <td><code>isfinite(interval)</code></td> 
   <td>boolean</td> 
   <td>Test for finite interval</td> 
   <td>isfinite(interval '4 hours')</td> 
   <td>true</td> 
  </tr> 
  <tr> 
   <td><code>justify_days(interval)</code></td> 
   <td>interval</td> 
   <td>Adjust interval so 30-day time periods are represented as months</td> 
   <td>justify_days(interval '35 days')</td> 
   <td>1 mon 5 days</td> 
  </tr> 
  <tr> 
   <td><code>justify_hours(interval)</code></td> 
   <td>interval</td> 
   <td>Adjust interval so 24-hour time periods are represented as days</td> 
   <td>justify_hours(interval '27 hours')</td> 
   <td>1 day 03:00:00</td> 
  </tr> 
  <tr> 
   <td><code>justify_interval(interval)</code></td> 
   <td>interval</td> 
   <td>Adjust interval using<span>&nbsp;<code>justify_days</code><span>&nbsp;and<span>&nbsp;<code>justify_hours</code>, with additional sign adjustments</span></span></span></td> 
   <td>justify_interval(interval '1 mon -1 hour')</td> 
   <td>29 days 23:00:00</td> 
  </tr> 
  <tr> 
   <td><code>localtime</code></td> 
   <td>time</td> 
   <td>Current time of day; see<span>&nbsp;<a href="http://www.postgresql.org/docs/9.1/static/functions-datetime.html#FUNCTIONS-DATETIME-CURRENT" rel="nofollow">Section 9.9.4</a></span></td> 
   <td>&nbsp;</td> 
   <td>&nbsp;</td> 
  </tr> 
  <tr> 
   <td><code>localtimestamp</code></td> 
   <td>timestamp</td> 
   <td>Current date and time (start of current transaction); see<span>&nbsp;<a href="http://www.postgresql.org/docs/9.1/static/functions-datetime.html#FUNCTIONS-DATETIME-CURRENT" rel="nofollow">Section 9.9.4</a></span></td> 
   <td>&nbsp;</td> 
   <td>&nbsp;</td> 
  </tr> 
  <tr> 
   <td><code>now()</code></td> 
   <td>timestamp with time zone</td> 
   <td>Current date and time (start of current transaction); see<span>&nbsp;<a href="http://www.postgresql.org/docs/9.1/static/functions-datetime.html#FUNCTIONS-DATETIME-CURRENT" rel="nofollow">Section 9.9.4</a></span></td> 
   <td>&nbsp;</td> 
   <td>&nbsp;</td> 
  </tr> 
  <tr> 
   <td><code>statement_timestamp()</code></td> 
   <td>timestamp with time zone</td> 
   <td>Current date and time (start of current statement); see<span>&nbsp;<a href="http://www.postgresql.org/docs/9.1/static/functions-datetime.html#FUNCTIONS-DATETIME-CURRENT" rel="nofollow">Section 9.9.4</a></span></td> 
   <td>&nbsp;</td> 
   <td>&nbsp;</td> 
  </tr> 
  <tr> 
   <td><code>timeofday()</code></td> 
   <td>text</td> 
   <td>Current date and time (like<span>&nbsp;<code>clock_timestamp</code>, but as a<span>&nbsp;text<span>&nbsp;string); see<a href="http://www.postgresql.org/docs/9.1/static/functions-datetime.html#FUNCTIONS-DATETIME-CURRENT" rel="nofollow">Section 9.9.4</a></span></span></span></td> 
   <td>&nbsp;</td> 
   <td>&nbsp;</td> 
  </tr> 
  <tr> 
   <td><code>transaction_timestamp()</code></td> 
   <td>timestamp with time zone</td> 
   <td>Current date and time (start of current transaction); see<span>&nbsp;<a href="http://www.postgresql.org/docs/9.1/static/functions-datetime.html#FUNCTIONS-DATETIME-CURRENT" rel="nofollow">Section 9.9.4</a></span></td> 
   <td>&nbsp;</td> 
   <td>&nbsp;</td> 
  </tr> 
 </tbody> 
</table>

In addition to these functions, the SQLOVERLAPS 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 intervalstart<=time<end, unlessstart andend 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 aninterval value to (or subtracting aninterval value from) atimestamp with time zone value, the days component advances (or decrements) the date of thetimestamp 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 meansinterval '1 day' does not necessarily equalinterval '24 hours'. For example, with the session time zone set toCST7CDT,timestamp with time zone '2005-04-02 12:00-07' + interval '1 day' will producetimestamp with time zone '2005-04-03 12:00-06', while addinginterval '24 hours' to the same initialtimestamp with time zone producestimestamp with time zone '2005-04-03 13:00-06', as there is a change in daylight saving time at2005-04-03 02:00 in time zoneCST7CDT.

Note there can be ambiguity in themonths 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 yield1 mon 1 day, while using May would yield1 mon 2 days because May has 31 days, while April has only 30.

## 9.9.1.`EXTRACT`,`date_part` ##

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 typetimestamp,time, orinterval. (Expressions of typedate are cast totimestamp 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 typedouble 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.

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

day

Fortimestamp values, the day (of the month) field (1 - 31) ; forinterval 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

Fordate andtimestamp values, the number of seconds since 1970-01-01 00:00:00 UTC (can be negative); forinterval 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

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

SELECT TIMESTAMP WITH TIME ZONE 'epoch' + 982384720.12 * INTERVAL '1 second';

(The`to_timestamp` function encapsulates the above conversion.)

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 todow except for Sunday. This matches theISO 8601 day of the week numbering.

isoyear

TheISO 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

EachISO 8601 week-numbering year begins with the Monday of the week containing the 4th of January, so in early January or late December theISO year may be different from the Gregorian year. See theweek 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.

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

Fortimestamp values, the number of the month within the year (1 - 12) ; forinterval 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[\[1\]][1])

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 usesUT1because 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 theISO 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, and2006-01-01 is part of the 52nd week of year 2005, while2012-12-31 is part of the first week of 2013. It's recommended to use theisoyear field together withweek 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 no0 AD, so subtractingBC years fromAD 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][].

The`date_part` function is modeled on the traditionalIngres equivalent to theSQL\-standard function`extract`:

date_part('field', source)

Note that here thefield 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

## 9.9.2.`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 typetimestamp orinterval. (Values of typedate andtime are cast automatically totimestamp orinterval, respectively.)field selects to which precision to truncate the input value. The return value is of typetimestamp orinterval with all fields that are less significant than the selected one set to zero (or one, for day and month).

Valid values forfield are:

<table> 
 <tbody> 
  <tr> 
   <td>microseconds</td> 
  </tr> 
  <tr> 
   <td>milliseconds</td> 
  </tr> 
  <tr> 
   <td>second</td> 
  </tr> 
  <tr> 
   <td>minute</td> 
  </tr> 
  <tr> 
   <td>hour</td> 
  </tr> 
  <tr> 
   <td>day</td> 
  </tr> 
  <tr> 
   <td>week</td> 
  </tr> 
  <tr> 
   <td>month</td> 
  </tr> 
  <tr> 
   <td>quarter</td> 
  </tr> 
  <tr> 
   <td>year</td> 
  </tr> 
  <tr> 
   <td>decade</td> 
  </tr> 
  <tr> 
   <td>century</td> 
  </tr> 
  <tr> 
   <td>millennium</td> 
  </tr> 
 </tbody> 
</table>

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

## 9.9.3.AT TIME ZONE ##

TheAT 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

<table>      
 <thead> 
  <tr> 
   <th>Expression</th> 
   <th>Return Type</th> 
   <th>Description</th> 
  </tr> 
 </thead> 
 <tbody> 
  <tr> 
   <td>timestamp without time zone<span>&nbsp;AT TIME ZONE<span>&nbsp;zone</span></span></td> 
   <td>timestamp with time zone</td> 
   <td>Treat given time stamp<span>&nbsp;<span>without time zone<span>&nbsp;as located in the specified time zone</span></span></span></td> 
  </tr> 
  <tr> 
   <td>timestamp with time zone<span>&nbsp;AT TIME ZONE<span>&nbsp;zone</span></span></td> 
   <td>timestamp without time zone</td> 
   <td>Convert given time stamp<span>&nbsp;<span>with time zone<span>&nbsp;to the new time zone, with no time zone designation</span></span></span></td> 
  </tr> 
  <tr> 
   <td>time with time zone<span>&nbsp;AT TIME ZONE<span>&nbsp;zone</span></span></td> 
   <td>time with time zone</td> 
   <td>Convert given time<span>&nbsp;<span>with time zone<span>&nbsp;to the new time zone</span></span></span></td> 
  </tr> 
 </tbody> 
</table>

In these expressions, the desired time zonezone 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 isPST8PDT):

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`timezone`(zone,timestamp) is equivalent to the SQL-conforming constructtimestamp AT TIME ZONEzone.

## 9.9.4. Current Date/Time ##

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.

> **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()
    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 historicalPostgreSQL function. Like`clock_timestamp()`, it returns the actual current time, but as a formattedtext string rather than atimestamp with time zone value.`now()` is a traditionalPostgreSQLequivalent to`transaction_timestamp()`.

All the date/time data types also accept the special literal valuenow 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

> **Tip:** You do not want to use the third form when specifying aDEFAULT clause while creating a table. The system will convertnow to atimestamp 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.

## 9.9.5. Delaying Execution ##

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

pg_sleep(seconds)

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

SELECT pg_sleep(1.5);

> **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.

<table> 
 <tbody> 
  <tr> 
   <td align="center">Warning</td> 
  </tr> 
  <tr> 
   <td align="left"> <p>Make sure that your session does not hold more locks than necessary when calling<code>pg_sleep</code>. Otherwise other sessions might have to wait for your sleeping process, slowing down the entire system.</p> </td> 
  </tr> 
 </tbody> 
</table>

### Notes ###

<table> 
 <tbody> 
  <tr> 
   <td align="left"><a href="http://www.postgresql.org/docs/9.1/static/functions-datetime.html#AEN13711" rel="nofollow"><span>[1]</span></a></td> 
   <td align="left"> <p>60 if leap seconds are implemented by the operating system</p> </td> 
  </tr> 
 </tbody> 
</table>

--------------------

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转载于:https://www.cnblogs.com/kungfupanda/p/4372252.html

[PostgreSQL]: /images/20211217/4eab279d92354ef18a3f1e454584f68a.png
[PostgreSQL 1]: http://www.postgresql.org/
[Documentation]: http://www.postgresql.org/docs
[Manuals]: http://www.postgresql.org/docs/manuals
[PostgreSQL 9.1]: http://www.postgresql.org/docs/9.1/static/index.html
[9.0]: http://www.postgresql.org/docs/9.0/static/functions-datetime.html
[9.2]: http://www.postgresql.org/docs/9.2/static/functions-datetime.html
[9.3]: http://www.postgresql.org/docs/9.3/static/functions-datetime.html
[9.4]: http://www.postgresql.org/docs/9.4/static/functions-datetime.html
[devel]: http://www.postgresql.org/docs/devel/static/functions-datetime.html
[7.1]: http://www.postgresql.org/docs/7.1/static/functions-datetime.html
[7.2]: http://www.postgresql.org/docs/7.2/static/functions-datetime.html
[7.3]: http://www.postgresql.org/docs/7.3/static/functions-datetime.html
[7.4]: http://www.postgresql.org/docs/7.4/static/functions-datetime.html
[8.0]: http://www.postgresql.org/docs/8.0/static/functions-datetime.html
[8.1]: http://www.postgresql.org/docs/8.1/static/functions-datetime.html
[8.2]: http://www.postgresql.org/docs/8.2/static/functions-datetime.html
[8.3]: http://www.postgresql.org/docs/8.3/static/functions-datetime.html
[8.4]: http://www.postgresql.org/docs/8.4/static/functions-datetime.html
[Table 9-28]: http://www.postgresql.org/docs/9.1/static/functions-datetime.html#FUNCTIONS-DATETIME-TABLE
[Table 9-27]: http://www.postgresql.org/docs/9.1/static/functions-datetime.html#OPERATORS-DATETIME-TABLE
[Section 9.8]: http://www.postgresql.org/docs/9.1/static/functions-formatting.html
[Section 8.5]: http://www.postgresql.org/docs/9.1/static/datatype-datetime.html
[1]: http://www.postgresql.org/docs/9.1/static/functions-datetime.html#FTN.AEN13711
[Table 9-29]: http://www.postgresql.org/docs/9.1/static/functions-datetime.html#FUNCTIONS-DATETIME-ZONECONVERT-TABLE
[Section 8.5.3]: http://www.postgresql.org/docs/9.1/static/datatype-datetime.html#DATATYPE-TIMEZONES
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