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This section describesPostgreSQL's functions for operating onsequence objects. Sequence objects (also called sequence generators or just sequences) are special single-row tables created withCREATE SEQUENCE. A sequence object is usually used to generate unique identifiers for rows of a table. The sequence functions, listed inTable 9-34, provide simple, multiuser-safe methods for obtaining successive sequence values from sequence objects.
Table 9-34. Sequence Functions
| Function | Return Type | Description |
|---|---|---|
nextval(text) | bigint | Advance sequence and return new value |
currval(text) | bigint | Return value most recently obtained withnextval |
setval(text,bigint) | bigint | Set sequence's current value |
setval(text,bigint,boolean) | bigint | Set sequence's current value andis_called flag |
For largely historical reasons, the sequence to be operated on by a sequence-function call is specified by a text-string argument. To achieve some compatibility with the handling of ordinarySQL names, the sequence functions convert their argument to lower case unless the string is double-quoted. Thus
nextval('foo')operates on sequencefoonextval('FOO')operates on sequencefoonextval('"Foo"')operates on sequenceFooThe sequence name can be schema-qualified if necessary:
nextval('myschema.foo')operates onmyschema.foonextval('"myschema".foo')same as abovenextval('foo')searches search path forfooOf course, the text argument can be the result of an expression, not only a simple literal, which is occasionally useful.
The available sequence functions are:
nextvalAdvance the sequence object to its next value and return that value. This is done atomically: even if multiple sessions executenextval concurrently, each will safely receive a distinct sequence value.
currvalReturn the value most recently obtained bynextval for this sequence in the current session. (An error is reported ifnextval has never been called for this sequence in this session.) Notice that because this is returning a session-local value, it gives a predictable answer even if other sessions are executingnextval meanwhile.
setvalReset the sequence object's counter value. The two-parameter form sets the sequence'slast_value field to the specified value and sets itsis_called field totrue, meaning that the nextnextval will advance the sequence before returning a value. In the three-parameter form,is_called may be set eithertrue orfalse. If it's set tofalse, the nextnextval will return exactly the specified value, and sequence advancement commences with the followingnextval. For example,
SELECT setval('foo', 42);Nextnextval will return 43SELECT setval('foo', 42, true);Same as aboveSELECT setval('foo', 42, false);Nextnextval will return 42The result returned bysetval is just the value of its second argument.
Important: To avoid blocking of concurrent transactions that obtain numbers from the same sequence, a
nextvaloperation is never rolled back; that is, once a value has been fetched it is considered used, even if the transaction that did thenextvallater aborts. This means that aborted transactions may leave unused"holes" in the sequence of assigned values.setvaloperations are never rolled back, either.
If a sequence object has been created with default parameters,nextval calls on it will return successive values beginning with 1. Other behaviors can be obtained by using special parameters in theCREATE SEQUENCE command; see its command reference page for more information.