CREATE [ TEMPORARY | TEMP ] SEQUENCE seqname [ INCREMENT increment ]
[ MINVALUE minvalue ] [ MAXVALUE maxvalue ]
[ START start ] [ CACHE cache ] [ CYCLE ]
If specified, the sequence object is created only for this session, and is automatically dropped on session exit. Existing permanent sequences with the same name are not visible (in this session) while the temporary sequence exists.
The name of a sequence to be created.
The INCREMENT increment clause is
optional. A positive value will make an ascending
sequence, a negative one a descending sequence. The
default value is one (1).
The optional clause MINVALUE
minvalue
determines the minimum value a sequence can generate. The
defaults are 1 and -2^63-1 for ascending and descending
sequences, respectively.
The optional clause MAXVALUE
maxvalue
determines the maximum value for the sequence. The
defaults are 2^63-1 and -1 for ascending and descending
sequences, respectively.
The optional START start clause enables the
sequence to begin anywhere. The default starting value is
minvalue for ascending
sequences and maxvalue
for descending ones.
The CACHE cache option enables
sequence numbers to be preallocated and stored in memory
for faster access. The minimum value is 1 (only one value
can be generated at a time, i.e., no cache) and this is
also the default.
The optional CYCLE keyword may be used to enable the
sequence to wrap around when the maxvalue or minvalue has been reached by an
ascending or descending sequence respectively. If the
limit is reached, the next number generated will be the
minvalue or maxvalue, respectively. Without
CYCLE, after the limit is reached nextval calls will return an error.
Message returned if the command is successful.
If the sequence specified already exists.
If the specified starting value is out of range.
If the specified starting value is out of range.
If the minimum and maximum values are inconsistent.
CREATE SEQUENCE will enter a new sequence number generator into the current database. This involves creating and initializing a new single-row table with the name seqname. The generator will be owned by the user issuing the command.
After a sequence is created, you use the functions
nextval, currval and setval to operate on the sequence. These
functions are documented in the User's
Guide.
Although you cannot update a sequence directly, you can use a query like
SELECT * FROM seqname;
to examine the parameters and current state of a sequence. In
particular, the last_value field of the
sequence shows the last value allocated by any backend process.
(Of course, this value may be obsolete by the time it's printed,
if other processes are actively doing nextval calls.)
| Caution |
|
Unexpected results may be obtained if a cache setting greater than one is
used for a sequence object that will be used concurrently
by multiple backends. Each backend will allocate and
cache successive sequence values during one access to the
sequence object and increase the sequence object's
last_value accordingly. Then,
the next cache-1 uses of
|
Use DROP SEQUENCE to remove a sequence.
Sequences are based on bigint arithmetic, so the range cannot exceed the range of an eight-byte integer (-9223372036854775808 to 9223372036854775807). On some older platforms, there may be no compiler support for eight-byte integers, in which case sequences use regular integer arithmetic (range -2147483648 to +2147483647).
When cache is greater than one, each backend uses its own cache to store preallocated numbers. Numbers that are cached but not used in the current session will be lost, resulting in "holes" in the sequence.
Create an ascending sequence called serial, starting at 101:
CREATE SEQUENCE serial START 101;
Select the next number from this sequence:
SELECT nextval('serial');
nextval
-------
114
Use this sequence in an INSERT:
INSERT INTO distributors VALUES (nextval('serial'), 'nothing');
Update the sequence value after a COPY FROM:
BEGIN;
COPY distributors FROM 'input_file';
SELECT setval('serial', max(id)) FROM distributors;
END;