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.
