Creating and Using Sequences

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Creating and Using Sequences

You can use sequences to auto-increment unique ID columns of a table whenever a record is added. Sequences are often used to assign unique identification numbers to rows added to a table. You can declare an identifier column of type SERIAL to implicitly create a sequence for use with a column.

Creating a Sequence

The CREATE SEQUENCE command creates and initializes a special single-row sequence generator table with the given sequence name. The sequence name must be distinct from the name of any other sequence, table, index, or view in the same schema. For example:


Using a Sequence

After you create a sequence generator table using CREATE SEQUENCE, you can use the nextval function to operate on the sequence. For example, to insert a row into a table that gets the next value of a sequence:

INSERT INTO vendors VALUES (nextval('myserial'), 'acme');

You can also use the setval function to reset a sequence's counter value. For example:

SELECT setval('myserial', 201);

A nextval operation is never rolled back. Afetched value is considered used, even if the transaction that performed the nextval fails. This means that failed transactions can leave unused holes in the sequence of assigned values. setval operations are never rolled back.

Note that the nextval function is not allowed in UPDATE or DELETE statements if mirroring is enabled, and the currval and lastval functions are not supported in Greenplum Database.

To examine the current settings of a sequence, query the sequence table:

SELECT * FROM myserial;

Altering a Sequence

The ALTER SEQUENCE command changes the parameters of an existing sequence generator. For example:


Any parameters not set in the ALTER SEQUENCE command retain their prior settings.

Dropping a Sequence

The DROP SEQUENCE command removes a sequence generator table. For example:


Making a Sequence the Default Value for a Column

A sequence can be used in the CREATE TABLE command (instead of the SERIAL or BIGSERIAL type):

            id INT4 DEFAULT nextval('id_seq')

Alternatively, a column can be altered and a sequence can be added as default value:

ALTER TABLE ... ALTER COLUMN id SET DEFAULT nextval('id_seq');