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 9-34, provide simple, multiuser-safe methods for obtaining successive sequence values from sequence objects.
Table 9-34. Sequence Functions
|bigint||Advance sequence and return new value|
|bigint||Return value most recently obtained with |
|bigint||Set sequence's current value|
|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 Foo
The 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 foo
Of 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:
Advance the sequence object to its next value and return that
value. This is done atomically: even if multiple sessions
nextval concurrently, each will safely receive
a distinct sequence value.
Return the value most recently obtained by
for this sequence in the current session. (An error is
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
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,
nextval will return exactly the specified
value, and sequence advancement commences with the following
nextval. For example,
SELECT setval('foo', 42); Next
nextvalwill return 43 SELECT setval('foo', 42, true); Same as above SELECT setval('foo', 42, false); Next
nextvalwill return 42
The result returned by
setval is just the value of its
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 the
nextvallater 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 the CREATE SEQUENCE command;
see its command reference page for more information.