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 ]
[ OWNED BY { table.column | 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 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 in the documentation.
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 might 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.
OWNED BY table.column
OWNED BY NONE
The OWNED BY option causes the sequence to be associated with a
specific table column, such that if that column (or its whole
table) is dropped, the sequence will be automatically dropped as
well. The specified table must have the same owner and be in the
same schema as the sequence. OWNED BY NONE, the default,
specifies that there is no such association.
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 might be no
compiler support for eight-byte integers, in which case sequences use
regular integer arithmetic (range -2147483648 to +2147483647).
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’);
nextval
---------
101
Select the next number from this sequence:
SELECT nextval(’serial’);
nextval
---------
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:
· The standard’s AS <data type> expression is not supported.
· Obtaining the next value is done using the nextval() function instead
of the standard’s NEXT VALUE FOR expression.
· The OWNED BY clause is a PostgreSQL extension.
SEE ALSO
ALTER SEQUENCE [alter_sequence(7)], DROP SEQUENCE [drop_sequence(7)]