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 nextval calls
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 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.
