This section describes functions for operating on sequence objects, also called sequence generators or just sequences. Sequence objects are special single-row tables created with CREATE SEQUENCE. Sequence objects are commonly used to generate unique identifiers for rows of a table. The sequence functions, listed in Table 9-40, provide simple, multiuser-safe methods for obtaining successive sequence values from sequence objects.
Table 9-40. Sequence Functions
|Return value most recently obtained with
nextval for specified sequence
|Return value most recently obtained with
nextval for any sequence
|Advance sequence and return new value
|Set sequence's current value
|Set sequence's current value and is_called flag
The sequence to be operated on by a sequence function is specified by a regclass argument, which is simply the OID of the sequence in the pg_class system catalog. You do not have to look up the OID by hand, however, since the regclass data type's input converter will do the work for you. Just write the sequence name enclosed in single quotes so that it looks like a literal constant. For compatibility with the handling of ordinary SQL names, the string will be converted to lower case unless it contains double quotes around the sequence name. 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
See Section 8.16 for more information about regclass.
Note: Before PostgreSQL 8.1, the arguments of the sequence functions were of type text, not regclass, and the above-described conversion from a text string to an OID value would happen at run time during each call. For backward compatibility, this facility still exists, but internally it is now handled as an implicit coercion from text to regclass before the function is invoked.
When you write the argument of a sequence function as an unadorned literal string, it becomes a constant of type regclass. Since this is really just an OID, it will track the originally identified sequence despite later renaming, schema reassignment, etc. This "early binding" behavior is usually desirable for sequence references in column defaults and views. But sometimes you might want "late binding" where the sequence reference is resolved at run time. To get late-binding behavior, force the constant to be stored as a text constant instead of regclass:nextval('foo'::text) foo is looked up at runtime
Note that late binding was the only behavior supported in PostgreSQL releases before 8.1, so you might need to do this to preserve the semantics of old applications.
Of course, the argument of a sequence function can be an expression as well as a constant. If it is a text expression then the implicit coercion will result in a run-time lookup.
The available sequence functions are:
Advance the sequence object to its next value and return
that value. This is done atomically: even if multiple
concurrently, each will safely receive a distinct sequence
If a sequence object has been created with default
nextval calls 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.
Important: To avoid blocking 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 might leave unused "holes" in the sequence of assigned values.
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.) Because this is returning a
session-local value, it gives a predictable answer whether
or not other sessions have executed
nextval since the current session
Return the value most recently returned by
nextval in the current session. This
function is identical to
currval, except that instead of taking
the sequence name as an argument it fetches the value of
the last sequence used by
nextval in the current session. It is an
error to call
nextval has not yet been
called in the current session.
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. The value reported by
currval is also set to the
specified value. In the three-parameter form, is_called can be set to either true or false.
true has the same effect as the
two-parameter form. If it is set to false, the next
nextval will return exactly the specified
value, and sequence advancement commences with the
Furthermore, the value reported by
currval is not changed in this case (this
is a change from pre-8.3 behavior). 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 second
Important: Because sequences are non-transactional, changes made by
setvalare not undone if the transaction rolls back.