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| PostgreSQL 8.1.4 Documentation | ||||
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CREATE [ TEMPORARY | TEMP ] SEQUENCE name [ INCREMENT [ BY ] increment ]
[ MINVALUE minvalue | NO MINVALUE ] [ MAXVALUE maxvalue | NO MAXVALUE ]
[ START [ WITH ] start ] [ CACHE cache ] [ [ NO ] CYCLE ]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.)
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.
The name (optionally schema-qualified) of the sequence to be created.
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.
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.
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.
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.
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.
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.
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.
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;