Table 9-44. Sequence Functions

Note: BeforePostgreSQL 8.1, the arguments of the sequence functions were of typetext, notregclass, 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 fromtext toregclass before the function is invoked.

When you write the argument of a sequence function as an unadorned literal string, it becomes a constant of typeregclass. 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 atext constant instead ofregclass:

nextval('foo'::text)foo is looked up at runtime

Note that late binding was the only behavior supported inPostgreSQL 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.

nextval

Advance the sequence object to its next value and return that value. This is done atomically: even if multiple sessions executenextval concurrently, each will safely receive a distinct sequence value.

If a sequence object has been created with default parameters, successivenextval calls will return successive values beginning with 1. Other behaviors can be obtained by using special parameters in theCREATE SEQUENCE command; see its command reference page for more information.

Important: To avoid blocking concurrent transactions that obtain numbers from the same sequence, anextval operation is never rolled back; that is, once a value has been fetched it is considered used, even if the transaction that did thenextval later aborts. This means that aborted transactions might leave unused"holes" in the sequence of assigned values.

currval

Return the value most recently obtained bynextval for this sequence in the current session. (An error is reported ifnextval 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 executednextval since the current session did.

lastval

Return the value most recently returned bynextval in the current session. This function is identical tocurrval, except that instead of taking the sequence name as an argument it fetches the value of the last sequence used bynextval in the current session. It is an error to calllastval ifnextval has not yet been called in the current session.

setval

Reset the sequence object's counter value. The two-parameter form sets the sequence'slast_value field to the specified value and sets itsis_called field totrue, meaning that the nextnextval will advance the sequence before returning a value. The value reported bycurrval is also set to the specified value. In the three-parameter form,is_called can be set to eithertrue orfalse.true has the same effect as the two-parameter form. If it is set tofalse, the nextnextval will return exactly the specified value, and sequence advancement commences with the followingnextval. Furthermore, the value reported bycurrval is not changed in this case. For example,

SELECT setval('foo', 42);Nextnextval will return 43SELECT setval('foo', 42, true);Same as aboveSELECT setval('foo', 42, false);Nextnextval will return 42

The result returned bysetval is just the value of its second argument.

Important: Because sequences are non-transactional, changes made bysetval are not undone if the transaction rolls back.