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 Tabela 9-34, provide simple, multiuser-safe methods for obtaining successive sequence values from sequence objects.
Tabela 9-34. Sequence Functions
|currval(regclass)||bigint||Return value most recently obtained with nextval for specified sequence|
|nextval(regclass)||bigint||Advance sequence and return new value|
|setval(regclass, bigint)||bigint||Set sequence's current value|
|setval(regclass, bigint, boolean)||bigint||Set sequence's current value and is_called flag|
The sequence to be operated on by a sequence-function call is specified by a regclass argument, which is just 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. To achieve some compatibility with the handling of ordinary SQL names, the string will be converted to lowercase 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 Seção 8.12 for more information about regclass.
Nota: 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 backwards 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 will 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 may 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 sessions execute nextval concurrently, each will safely receive a distinct sequence value.
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 whether or not other sessions have executed nextval since the current session did.
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 that nextval was used on in the current session. It is an error to call lastval if 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. 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.
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.
Importante: 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.