CREATE SEQUENCE

CREATE SEQUENCE

7

SQL - Language Statements

CREATE SEQUENCE

define a new sequence generator

CREATE \[ { TEMPORARY \| TEMP } \| UNLOGGED \] SEQUENCE \[ IF NOT EXISTS
\]

name

\[ AS

data\_type

\] \[ INCREMENT \[ BY \]

increment

\] \[ MINVALUE

minvalue

\| NO MINVALUE \] \[ MAXVALUE

maxvalue

\| NO MAXVALUE \] \[ START \[ WITH \]

start

\] \[ CACHE

cache

\] \[ \[ NO \] CYCLE \] \[ OWNED BY {

table\_name

.

column\_name

\| NONE } \]

Description
===========

`CREATE SEQUENCE` creates a new sequence number generator. This involves
creating and initializing a new special single-row table with the name
name. 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. Temporary
sequences exist in a special schema, so a schema name cannot be given
when creating a temporary sequence. The sequence name must be distinct
from the name of any other relation (table, sequence, index, view,
materialized view, or foreign table) 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 [???](#functions-sequence).

Although you cannot update a sequence directly, you can use a query
like:

    SELECT * FROM name;

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 session. (Of course, this value might be obsolete by
the time it\'s printed, if other sessions are actively doing `nextval`
calls.)

Parameters
==========

`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.

`UNLOGGED`

:   If specified, the sequence is created as an unlogged sequence.
    Changes to unlogged sequences are not written to the write-ahead
    log. They are not crash-safe: an unlogged sequence is automatically
    reset to its initial state after a crash or unclean shutdown.
    Unlogged sequences are also not replicated to standby servers.

    Unlike unlogged tables, unlogged sequences do not offer a
    significant performance advantage. This option is mainly intended
    for sequences associated with unlogged tables via identity columns
    or serial columns. In those cases, it usually wouldn\'t make sense
    to have the sequence WAL-logged and replicated but not its
    associated table.

`IF NOT EXISTS`

:   Do not throw an error if a relation with the same name already
    exists. A notice is issued in this case. Note that there is no
    guarantee that the existing relation is anything like the sequence
    that would have been created MDASH it might not even be a sequence.

name

:   The name (optionally schema-qualified) of the sequence to be
    created.

data\_type

:   The optional clause `AS data_type` specifies the data type of the
    sequence. Valid types are `smallint`, `integer`, and `bigint`.
    `bigint` is the default. The data type determines the default
    minimum and maximum values of the sequence.

increment

:   The optional clause `INCREMENT BY increment` specifies which value
    is added to the current sequence value to create a new value. A
    positive value will make an ascending sequence, a negative one a
    descending sequence. The default value is 1.

minvalue; `NO MINVALUE`

:   The optional clause `MINVALUE minvalue` determines the minimum value
    a sequence can generate. If this clause is not supplied or
    `NO MINVALUE` is specified, then defaults will be used. The default
    for an ascending sequence is 1. The default for a descending
    sequence is the minimum value of the data type.

maxvalue; `NO MAXVALUE`

:   The optional clause `MAXVALUE maxvalue` determines the maximum value
    for the sequence. If this clause is not supplied or `NO MAXVALUE` is
    specified, then default values will be used. The default for an
    ascending sequence is the maximum value of the data type. The
    default for a descending sequence is -1.

start

:   The optional clause `START WITH start ` allows the sequence to begin
    anywhere. The default starting value is minvalue for ascending
    sequences and maxvalue for descending ones.

cache

:   The optional clause `CACHE cache` specifies how many sequence
    numbers are 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`; `NO CYCLE`

:   The `CYCLE` option allows 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.

    If `NO CYCLE` is specified, any calls to `nextval` after the
    sequence has reached its maximum value will return an error. If
    neither `CYCLE` or `NO CYCLE` are specified, `NO CYCLE` is the
    default.

`OWNED BY` table\_name.column\_name; `OWNED BY NONE`

:   The `OWNED BY` option causes the sequence to be associated with a
    specific table column, such that if that column (or its whole table)
    is dropped, the sequence will be automatically dropped as well. The
    specified table must have the same owner and be in the same schema
    as the sequence. `OWNED BY NONE`, the default, specifies that there
    is no such association.

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).

Because `nextval` and `setval` calls are never rolled back, sequence
objects cannot be used if "gapless" assignment of sequence numbers is
needed. It is possible to build gapless assignment by using exclusive
locking of a table containing a counter; but this solution is much more
expensive than sequence objects, especially if many transactions need
sequence numbers concurrently.

Unexpected results might be obtained if a cache setting greater than one
is used for a sequence object that will be used concurrently by multiple
sessions. Each session 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 session simply return the preallocated values
without touching the sequence object. So, any numbers allocated but not
used within a session will be lost when that session ends, resulting in
"holes" in the sequence.

Furthermore, although multiple sessions are guaranteed to allocate
distinct sequence values, the values might be generated out of sequence
when all the sessions are considered. For example, with a cache setting
of 10, session A might reserve values 1..10 and return `nextval`=1, then
session B might reserve values 11..20 and return `nextval`=11 before
session 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 session, 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 sessions until they have used up any
preallocated values they have cached.

Examples
========

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
    ---------
         101

Select the next number from this sequence:

    SELECT nextval('serial');

     nextval
    ---------
         102

Use this sequence in an `INSERT` command:

    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
=============

`CREATE SEQUENCE` conforms to the SQL standard, with the following
exceptions:

-   Obtaining the next value is done using the `nextval()` function
    instead of the standard\'s `NEXT VALUE FOR` expression.

-   The `OWNED BY` clause is a PostgreSQL extension.

See Also
========
