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CREATE SEQUENCE

Name

CREATE SEQUENCE  --  Creates a new sequence number generator

Synopsis

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

The optional clause MAXVALUE maxvalue determines 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 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.

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 data base. 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 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 seqname;
   
to examine the parameters of a sequence. As an alternative to fetching the parameters from the original definition as above, you can use
SELECT last_value FROM seqname;
   
to obtain the last value allocated by any backend.

To avoid blocking of concurrent transactions that obtain numbers from the same sequence, a nextval operation is never rolled back; that is, once a value has been fetched it is considered used, even if the transaction that did the nextval later aborts. This means that aborted transactions may leave unused "holes" in the sequence of assigned values. setval operations are never rolled back, either.

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, 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. 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.

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');
   

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

SQL92

CREATE SEQUENCE is a Postgres language extension. There is no CREATE SEQUENCE statement in SQL92.

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