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CREATE
NAME
CREATE SEQUENCE - define a new sequence generator
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.
seqname
The name 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 nextval calls will return an
error.
OUTPUTS
CREATE
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.
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 nextval within that backend simply return the preallocated
values without touching the shared object. So, any numbers allocated
but not used within a session will be lost when that session ends.
Furthermore, although multiple backends are guaranteed to allocate
distinct sequence values, the values may be generated out of sequence
when all the backends are considered. (For example, with a cache
setting of 10, backend A might reserve values 1..10 and return
nextval=1, then backend B might reserve values 11..20 and return
nextval=11 before backend 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 backend, 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 backends
until they have used up any preallocated values they have cached.
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;
COMPATIBILITY
SQL92
CREATE SEQUENCE is a PostgreSQL language extension. There is no CREATE
SEQUENCE statement in SQL92.
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