CREATE SEQUENCE
CREATE SEQUENCE
Synopsis
CREATE [ TEMPORARY | TEMP ] SEQUENCE seqname [ INCREMENT increment ]
[ MINVALUE minvalue ] [ MAXVALUE maxvalue ]
[ START start ] [ CACHE cache ] [ CYCLE ]
Inputs
- 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.
- seqname
The name (optionally schema-qualified) of a sequence to be created.
- increment
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).
- minvalue
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.
- maxvalue
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.
- start
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.
- cache
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.
- CYCLE
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
nextvalcalls will return an error.
Outputs
- CREATE SEQUENCE
Message returned if the command is successful.
- ERROR: Relation 'seqname' already exists
If the sequence specified already exists.
- ERROR: DefineSequence: MINVALUE (start) can't be >= MAXVALUE (max)
If the specified starting value is out of range.
- ERROR: DefineSequence: START value (start) can't be < MINVALUE (min)
If the specified starting value is out of range.
- ERROR: DefineSequence: MINVALUE (min) can't be >= MAXVALUE (max)
If the minimum and maximum values are inconsistent.
Description
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.
If a schema name is given then the sequence is created in the specified schema. Otherwise it is created in the current schema (the one at the front of the search path; see CURRENT_SCHEMA()). TEMP sequences exist in a special schema, so a schema name may not be given when creating a TEMP 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
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 |
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).
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.
Usage
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;