create_sequence(7)

NAME#

CREATE_SEQUENCE - define a new sequence generator

SYNOPSIS#

CREATE [ { TEMPORARY | TEMP } | UNLOGGED ] SEQUENCE [ IF NOT EXISTS ] \nname\n
\n
    [ AS \ndata_type\n ]
\n
    [ INCREMENT [ BY ] \nincrement\n ]
\n
    [ MINVALUE \nminvalue\n | NO MINVALUE ] [ MAXVALUE \nmaxvalue\n | NO MAXVALUE ]
\n
    [ START [ WITH ] \nstart\n ] [ CACHE \ncache\n ] [ [ NO ] CYCLE ]
\n
    [ OWNED BY { \ntable_name\n.\ncolumn_name\n | NONE } ]

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 cannot be given when creating a temporary sequence. The sequence name must be distinct from the name of any other relation (table, sequence, index, view, materialized view, or foreign table) 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 9.17.

Although you cannot update a sequence directly, you can use a query like:

SELECT * FROM \nname\n;

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 might be obsolete by the time it's printed, if other sessions are actively doing nextval calls.)

PARAMETERS#

TEMPORARY or TEMP

UNLOGGED

Unlike unlogged tables, unlogged sequences do not offer a significant performance advantage. This option is mainly intended for sequences associated with unlogged tables via identity columns or serial columns. In those cases, it usually wouldn't make sense to have the sequence WAL-logged and replicated but not its associated table.

IF NOT EXISTS

name

data_type

increment

minvalue NO MINVALUE

maxvalue NO MAXVALUE

start

cache

CYCLE NO CYCLE

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.

OWNED BY table_name.column_name OWNED BY NONE

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).

Because nextval and setval calls are never rolled back, sequence objects cannot be used if “gapless” assignment of sequence numbers is needed. It is possible to build gapless assignment by using exclusive locking of a table containing a counter; but this solution is much more expensive than sequence objects, especially if many transactions need sequence numbers concurrently.

Unexpected results might 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 might 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');
\n
 nextval
---------
\n
     101

Select the next number from this sequence:

SELECT nextval('serial');
\n
 nextval
---------
\n
     102

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:

SEE ALSO#

ALTER SEQUENCE (ALTER_SEQUENCE(7)), DROP SEQUENCE (DROP_SEQUENCE(7))