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

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,

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.