9.9 日期時間函式及運算子
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 |
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Table 9.31. Date/Time Functions
| Function | Return Type | Description | Example | Result |
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| Subtract arguments, producing a “symbolic” result that uses years and months, rather than just days |
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| Subtract from |
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| Current date and time (changes during statement execution); see Section 9.9.4 | ||
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| Current date; see Section 9.9.4 | ||
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| Current time of day; see Section 9.9.4 | ||
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| Current date and time (start of current transaction); see Section 9.9.4 | ||
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| Get subfield (equivalent to |
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| Get subfield (equivalent to |
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| Truncate to specified precision; see Section 9.9.2 |
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| Truncate to specified precision in the specified time zone; see Section 9.9.2 |
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| Truncate to specified precision; see Section 9.9.2 |
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| Get subfield; see Section 9.9.1 |
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| Get subfield; see Section 9.9.1 |
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| Test for finite date (not +/-infinity) |
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| Test for finite time stamp (not +/-infinity) |
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| Test for finite interval |
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| Adjust interval so 30-day time periods are represented as months |
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| Adjust interval so 24-hour time periods are represented as days |
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| Adjust interval using |
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| Current time of day; see Section 9.9.4 | ||
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| Current date and time (start of current transaction); see Section 9.9.4 | ||
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| Create date from year, month and day fields |
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| Create interval from years, months, weeks, days, hours, minutes and seconds fields |
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| Create time from hour, minute and seconds fields |
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| Create timestamp from year, month, day, hour, minute and seconds fields |
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| Create timestamp with time zone from year, month, day, hour, minute and seconds fields; if |
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| Current date and time (start of current transaction); see Section 9.9.4 | ||
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| Current date and time (start of current statement); see Section 9.9.4 | ||
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| Current date and time (like | ||
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| Current date and time (start of current transaction); see Section 9.9.4 | ||
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| Convert Unix epoch (seconds since 1970-01-01 00:00:00+00) to timestamp |
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In addition to these functions, the SQL OVERLAPS operator is supported:
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.
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, keeping the time of day the same. 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 America/Denver:
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:
9.9.1. EXTRACT, date_part
EXTRACT, date_partThe 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:century
The century
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
decade
The year field divided by 10
dow
The day of the week as Sunday (0) to Saturday (6)
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)
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
You can convert an epoch value back to a time stamp with to_timestamp:
hour
The hour field (0 - 23)
isodow
The day of the week as Monday (1) to Sunday (7)
This is identical to dow except for Sunday. This matches the ISO 8601 day of the week numbering.isoyear
The ISO 8601 week-numbering year that the date falls in (not applicable to intervals)
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.microseconds
The seconds field, including fractional parts, multiplied by 1 000 000; note that this includes full seconds
millennium
The millennium
Years in the 1900s are in the second millennium. The third millennium started January 1, 2001.milliseconds
The seconds field, including fractional parts, multiplied by 1000. Note that this includes full seconds.
minute
The minutes field (0 - 59)
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)
quarter
The quarter of the year (1 - 4) that the date is in
second
The seconds field, including fractional parts (0 - 59[7])
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 does not use UTC because leap seconds are not handled.)timezone_hour
The hour component of the time zone offsettimezone_minute
The minute component of the time zone offsetweek
The 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.
year
The year field. Keep in mind there is no 0 AD, so subtracting BC years from AD years should be done with care.
Note
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:
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.
9.9.2. date_trunc
date_truncThe function date_trunc is conceptually similar to the trunc function for numbers.
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:
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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):
9.9.3. AT TIME ZONE
AT TIME ZONEThe 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
AT TIME ZONE Variants| Expression | Return Type | Description |
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| Treat given time stamp without time zone as located in the specified time zone |
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| Convert given time stamp with time zone to the new time zone, with no time zone designation |
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| 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):
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) is equivalent to the SQL-conforming construct timestamp AT TIME ZONE zone.
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_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:
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() 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:
Tip
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.
9.9.5. Delaying Execution
The following functions are available to delay execution of the server process:
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:
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. In particular, pg_sleep_until is not guaranteed to wake up exactly at the specified time, but it will not wake up any earlier.
Warning
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.\
[7] 60 if leap seconds are implemented by the operating system
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