Table 9.31 shows the available
   functions for date/time value processing, with details appearing in
   the following subsections.  Table 9.30 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 9.30. 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 9.31. 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 Section 9.9.2 | date_trunc('hour', timestamp '2001-02-16 20:38:40') | 2001-02-16 20:00:00 | 
|  | timestamp with time zone | Truncate to specified precision in the specified time zone; see Section 9.9.2 | date_trunc('day', timestamptz '2001-02-16 20:38:40+00', 'Australia/Sydney') | 2001-02-16 13:00:00+00 | 
|  | interval | Truncate to specified precision; see 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_daysandjustify_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 timezoneis 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 atextstring);
         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_partEXTRACT(fieldFROMsource)
    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:
    
    
centuryThe century
SELECT EXTRACT(CENTURY FROM TIMESTAMP '2000-12-16 12:21:13'); Result:20SELECT 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: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 (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:982384720.12SELECT 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
hourThe 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.
       
isoyearThe ISO 8601 week-numbering year that the date falls in (not applicable to intervals)
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.
       
This field is not available in PostgreSQL releases prior to 8.3.
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
millenniumThe 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.
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
minuteThe 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: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 fractional parts (0 - 59[7])
SELECT EXTRACT(SECOND FROM TIMESTAMP '2001-02-16 20:38:40'); Result:40SELECT EXTRACT(SECOND FROM TIME '17:12:28.5'); Result:28.5
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 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[,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
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 9.32 shows its variants.
   
Table 9.32. 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 19:38:40-08SELECT 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 20:38:40-05' 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 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:14:39:53.662522-05SELECT CURRENT_DATE; Result:2001-12-23SELECT CURRENT_TIMESTAMP; Result:2001-12-23 14:39:53.662522-05SELECT CURRENT_TIMESTAMP(2); Result:2001-12-23 14:39:53.66-05SELECT 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.