CREATE SEQUENCE

SQL - Language Statements

CREATE SEQUENCE

define a new sequence generator

1999-07-20

    CREATE [ TEMPORARY | TEMP ] SEQUENCE seqname [ INCREMENT increment ]
        [ MINVALUE minvalue ] [ MAXVALUE maxvalue ]
        [ START start ] [ CACHE cache ] [ CYCLE ]

1998-09-11

## 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, 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.

1998-09-11

## 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.

1998-09-11

## 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.)

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.

1998-09-11

### 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

1998-09-11

### SQL92

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