This section describes PostgreSQL's functions for operating on sequence objects. Sequence objects (also called sequence generators or just sequences) are special single-row tables created with CREATE SEQUENCE. A sequence object is usually used to generate unique identifiers for rows of a table. The sequence functions, listed in Table 6-26, provide simple, multiuser-safe methods for obtaining successive sequence values from sequence objects.
Table 6-26. Sequence Functions
| Function | Returns | Description |
|---|---|---|
nextval(text) | bigint | Advance sequence and return new value |
currval(text) | bigint | Return value most recently obtained with nextval |
setval(text,bigint) | bigint | Set sequence's current value |
setval(text,bigint,boolean) | bigint | Set sequence's current value and is_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 ordinary SQL names, the sequence functions convert their argument to lower case unless the string is double-quoted. Thus
nextval('foo') operates on sequence foo
nextval('FOO') operates on sequence foo
nextval('"Foo"') operates on sequence FooThe sequence name can be schema-qualified if necessary:
nextval('myschema.foo') operates on myschema.foo
nextval('"myschema".foo') same as above
nextval('foo') searches search path for fooOf 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:
nextval Advance the sequence object to its next value and return that value. This is done atomically: even if multiple sessions execute nextval concurrently, each will safely receive a distinct sequence value.
currval Return the value most recently obtained by nextval for this sequence in the current session. (An error is reported if nextval 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 executing nextval meanwhile.
setval Reset the sequence object's counter value. The two-parameter form sets the sequence's last_value field to the specified value and sets its is_called field to true, meaning that the next nextval will advance the sequence before returning a value. In the three-parameter form, is_called may be set either true or false. If it's set to false, the next nextval will return exactly the specified value, and sequence advancement commences with the following nextval. For example,
SELECT setval('foo', 42); Next nextval() will return 43
SELECT setval('foo', 42, true); Same as above
SELECT setval('foo', 42, false); Next nextval() will return 42 The result returned by setval 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 one. Other behaviors can be obtained by using special parameters in the CREATE SEQUENCE command; see its command reference page for more information.
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