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 nextval within that backend simply return the preallocated values without touching the shared object. So, any numbers allocated but not used within a session will be lost when that session ends. Furthermore, although multiple backends are guaranteed to allocate distinct sequence values, the values may be generated out of sequence when all the backends are considered. (For example, with a cache setting of 10, backend A might reserve values 1..10 and return nextval=1, then backend B might reserve values 11..20 and return nextval=11 before backend A has generated nextval=2.) Thus, with a cache setting of one it is safe to assume that nextval values are generated sequentially; with a cache setting greater than one you should only assume that the nextval values are all distinct, not that they are generated purely sequentially. Also, last_value will reflect the latest value reserved by any backend, whether or not it has yet been returned by nextval. Another consideration is that a setval executed on such a sequence will not be noticed by other backends until they have used up any preallocated values they have cached. |
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;