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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 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:
- 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 nextval operation is never rolled back;
that is, once a value has been fetched it is considered used, even if the transaction that
did the nextval later aborts. This means that aborted
transactions may leave unused "holes" in the sequence
of assigned values. setval operations 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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