CREATE SEQUENCE
PostgreSQL User's GuidePrevNextCREATE SEQUENCEName CREATE SEQUENCE
— Creates a new sequence number generator
CREATE SEQUENCE seqname
[ INCREMENT increment ]
[ MINVALUE minvalue ]
[ MAXVALUE maxvalue ]
[ START start ]
[ CACHE cache ]
[ CYCLE ]
Inputs
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 -2147483647 for
ascending and descending sequences, respectively.
maxvalue Use the optional clause MAXVALUE
maxvalue to
determine the maximum
value for the sequence. The defaults are 2147483647 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 continue 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 whatever the
minvalue or
maxvalue is,
as appropriate.
Outputs
CREATE Message returned if the command is successful.
ERROR: amcreate: 'seqname' relation already exists If the sequence specified already exists.
ERROR: DefineSequence: START value (start) can't be > MAXVALUE (maxvalue) If the specified starting value is out of range.
ERROR: DefineSequence: START value (start) can't be < MINVALUE (minvalue) If the specified starting value is out of range.
ERROR: DefineSequence: MINVALUE (minvalue) can't be >= MAXVALUE (maxvalue) If the minimum and maximum values are inconsistant.
Description
CREATE SEQUENCE will enter a new sequence number generator
into the current data base. This involves creating and initialising 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 may use the function
nextval(seqname)
to get a new number from the sequence.
The function
currval('seqname')
may be used to determine the number returned by the last call to
nextval(seqname)
for the specified sequence in the current session.
The function
setval('seqname',
newvalue)
may be used to set the current value of the specified sequence.
The next call to
nextval(seqname)
will return the given value plus the sequence increment.
Use a query like
SELECT * FROM sequence_name;
to get the parameters of a sequence.
Aside from fetching the original
parameters, you can use
SELECT last_value FROM sequence_name;
to obtain the last value allocated by any backend.
parameters, you can use
Low-level locking is used to enable multiple simultaneous
calls to a generator.
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 "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, numbers allocated but not used in the current session
will be lost. 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. Notes
Refer to the DROP SEQUENCE statement to remove a sequence.
Each backend uses its own cache to store allocated 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');
Set the sequence value after a COPY FROM:
CREATE FUNCTION distributors_id_max() RETURNS INT4
AS 'SELECT max(id) FROM distributors'
LANGUAGE 'sql';
BEGIN;
COPY distributors FROM 'input_file';
SELECT setval('serial', distributors_id_max());
END;
Compatibility
CREATE SEQUENCE is a Postgres
language extension.
SQL92
There is no CREATE SEQUENCE statement
in SQL92.
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