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Note: The following description applies both to Postgres-XC and PostgreSQL if not described explicitly.
This section describes Postgres-XC'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 9-39, provide simple, multiuser-safe methods for obtaining successive sequence values from sequence objects.
Table 9-39. Sequence Functions
| Function | Return Type | Description |
|---|---|---|
currval(regclass) | bigint | Return value most recently obtained with
nextval for specified sequence |
lastval() | bigint | Return value most recently obtained with
nextval for any 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 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 FooThe 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 fooSee 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 runtimeNote 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:
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.) 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.
lastval 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
lastval if nextval
has not yet been called in the current session.
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. 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 following
nextval. 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 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,
successive 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 thenextvallater aborts. This means that aborted transactions might leave unused "holes" in the sequence of assigned values.setvaloperations are never rolled back, either.