CREATE SEQUENCE
Name
CREATE SEQUENCE -- define a new sequence generator
Synopsis
CREATE [ TEMPORARY | TEMP ] SEQUENCE
name
[ INCREMENT [ BY ]
increment
]
[ MINVALUE
minvalue
| NO MINVALUE ] [ MAXVALUE
maxvalue
| NO MAXVALUE ]
[ START [ WITH ]
start
] [ CACHE
cache
] [ [ NO ] CYCLE ]
Description
CREATE SEQUENCE creates a new sequence number generator. This involves creating and initializing a new special single-row table with the name
name
. The generator will be owned by the user issuing the command.
If a schema name is given then the sequence is created in the specified schema. Otherwise it is created in the current schema. Temporary sequences exist in a special schema, so a schema name may not be given when creating a temporary sequence. The sequence name must be distinct from the name of any other sequence, table, index, or view in the same schema.
After a sequence is created, you use the functions nextval, currval, and setval to operate on the sequence. These functions are documented in Section 9.12.
Although you cannot update a sequence directly, you can use a query like
SELECT * FROM
name
;
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 session. (Of course, this value may be obsolete by the time it's printed, if other sessions are actively doing nextval calls.)
Parameters
-
TEMPORARY or TEMP
-
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, unless they are referenced with schema-qualified names.
-
name
-
The name (optionally schema-qualified) of the sequence to be created.
-
increment
-
The optional clause INCREMENT BY
increment
specifies which value is added to the current sequence value to create a new value. A positive value will make an ascending sequence, a negative one a descending sequence. The default value is 1.
-
minvalue
NO MINVALUE
-
The optional clause MINVALUE
minvalue
determines the minimum value a sequence can generate. If this clause is not supplied or NO MINVALUE is specified, then defaults will be used. The defaults are 1 and -263-1 for ascending and descending sequences, respectively.
-
maxvalue
NO MAXVALUE
-
The optional clause MAXVALUE
maxvalue
determines the maximum value for the sequence. If this clause is not supplied or NO MAXVALUE is specified, then default values will be used. The defaults are 263-1 and -1 for ascending and descending sequences, respectively.
-
start
-
The optional clause START WITH
start
allows the sequence to begin anywhere. The default starting value is
minvalue
for ascending sequences and
maxvalue
for descending ones.
-
cache
-
The optional clause CACHE
cache
specifies how many sequence numbers are 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.
-
CYCLE
NO CYCLE
-
The CYCLE option allows 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.
If NO CYCLE is specified, any calls to nextval after the sequence has reached its maximum value will return an error. If neither CYCLE or NO CYCLE are specified, NO CYCLE is the default.
Notes
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).
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 sessions. Each session 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 session simply return the preallocated values without touching the sequence object. So, any numbers allocated but not used within a session will be lost when that session ends, resulting in "holes" in the sequence.
Furthermore, although multiple sessions are guaranteed to allocate distinct sequence values, the values may be generated out of sequence when all the sessions are considered. For example, with a
cache
setting of 10, session A might reserve values 1..10 and return nextval=1, then session B might reserve values 11..20 and return nextval=11 before session 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 session, 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 sessions until they have used up any preallocated values they have cached.
Examples
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 command:
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;
Compatibility
CREATE SEQUENCE conforms to the SQL standard, with the following exceptions:
