8.8. Date/Time Functions and Operators

8.8.1. EXTRACT, date_part

EXTRACT (field FROM source)

The extract function retrieves subfields from date/time values, such as year or hour. source is a value expression that evaluates to type TIMESTAMP or interval. (Expressions of type DATE or time 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:

The extract function is primarily intended for computational processing. For formatting date/time values for display, see Section 8.7.

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') FROM DUAL;
Result: 16

SELECT date_part('hour', INTERVAL '4 hours 3 minutes') FROM DUAL;
Result: 4

8.8.2. TRUNC

The TRUNC function returns date with the time portion of the day truncated to the unit specified by the format model fmt. The value of the returned datatype is TIMESTAMP. Please refer to Table 8-27 for the permitted format models to use in fmt.

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').

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 zone-less time stamp and interprets it as MST time (UTC-7) to produce a UTC time stamp, which is then rotated 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 construct timestamp AT TIME ZONE zone.

8.8.3. date_trunc

The function date_trunc is conceptually similar to the trunc function for numbers.

date_trunc('field', source)

source is a value expression of type TIMESTAMP or interval. (Values of type DATE and time are cast automatically, to TIMESTAMP or interval respectively.) field selects to which precision to truncate the input value. The return value is of type TIMESTAMP or interval with all fields that are less significant than the selected one set to zero (or one, for day and month).

Valid values for field are:

Examples:

SELECT date_trunc('hour', TIMESTAMP '2001-02-16 20:38:40') FROM DUAL;
Result: 2001-02-16 20:00:00

SELECT date_trunc('year', TIMESTAMP '2001-02-16 20:38:40') FROM DUAL;
Result: 2001-01-01 00:00:00

8.8.4. Current Date/Time

EntrepriseDB 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_TIMESTAMP
CURRENT_TIMESTAMP(precision )
LOCALTIMESTAMP
LOCALTIMESTAMP(precision )
SYSDATE

CURRENT_TIMESTAMP, 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_DATE FROM DUAL;
Result: 2001-12-23

SELECT CURRENT_TIMESTAMP FROM DUAL;
Result: 2001-12-23 14:39:53.662522-05

SELECT CURRENT_TIMESTAMP(2) FROM DUAL;
Result: 2001-12-23 14:39:53.66-05

SELECT LOCALTIMESTAMP FROM DUAL;
Result: 2001-12-23 14:39:53.662522

SELECT SYSDATE FROM DUAL; 
Result: 2001-12-23 14:39:53

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. timeofday() returns the wall-clock time and does advance during transactions.

8.8.5. 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 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);