path: root/doc/src/sgml/man7/CREATE_SEQUENCE.7

'\" t
.\"     Title: CREATE SEQUENCE
.\"    Author: The PostgreSQL Global Development Group
.\" Generator: DocBook XSL Stylesheets vsnapshot <http://docbook.sf.net/>
.\"      Date: 2024
.\"    Manual: PostgreSQL 15.7 Documentation
.\"    Source: PostgreSQL 15.7
.\"  Language: English
.\"
.TH "CREATE SEQUENCE" "7" "2024" "PostgreSQL 15.7" "PostgreSQL 15.7 Documentation"
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.SH "NAME"
CREATE_SEQUENCE \- define a new sequence generator
.SH "SYNOPSIS"
.sp
.nf
CREATE [ { TEMPORARY | TEMP } | UNLOGGED ] SEQUENCE [ IF NOT EXISTS ] \fIname\fR
    [ AS \fIdata_type\fR ]
    [ INCREMENT [ BY ] \fIincrement\fR ]
    [ MINVALUE \fIminvalue\fR | NO MINVALUE ] [ MAXVALUE \fImaxvalue\fR | NO MAXVALUE ]
    [ START [ WITH ] \fIstart\fR ] [ CACHE \fIcache\fR ] [ [ NO ] CYCLE ]
    [ OWNED BY { \fItable_name\fR\&.\fIcolumn_name\fR | NONE } ]
.fi
.SH "DESCRIPTION"
.PP
\fBCREATE SEQUENCE\fR
creates a new sequence number generator\&. This involves creating and initializing a new special single\-row table with the name
\fIname\fR\&. The generator will be owned by the user issuing the command\&.
.PP
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\&.
.PP
After a sequence is created, you use the functions
\fBnextval\fR,
\fBcurrval\fR, and
\fBsetval\fR
to operate on the sequence\&. These functions are documented in
Section\ \&9.17\&.
.PP
Although you cannot update a sequence directly, you can use a query like:
.sp
.if n \{\
.RS 4
.\}
.nf
SELECT * FROM \fIname\fR;
.fi
.if n \{\
.RE
.\}
.sp
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\*(Aqs printed, if other sessions are actively doing
\fBnextval\fR
calls\&.)
.SH "PARAMETERS"
.PP
TEMPORARY or TEMP
.RS 4
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\&.
.RE
.PP
UNLOGGED
.RS 4
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\&.
.sp
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\*(Aqt make sense to have the sequence WAL\-logged and replicated but not its associated table\&.
.RE
.PP
IF NOT EXISTS
.RS 4
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 \(em it might not even be a sequence\&.
.RE
.PP
\fIname\fR
.RS 4
The name (optionally schema\-qualified) of the sequence to be created\&.
.RE
.PP
\fIdata_type\fR
.RS 4
The optional clause
AS \fIdata_type\fR
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\&.
.RE
.PP
\fIincrement\fR
.RS 4
The optional clause
INCREMENT BY \fIincrement\fR
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\&.
.RE
.PP
\fIminvalue\fR
.br
NO MINVALUE
.RS 4
The optional clause
MINVALUE \fIminvalue\fR
determines the minimum value a sequence can generate\&. If this clause is not supplied or
\fBNO MINVALUE\fR
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\&.
.RE
.PP
\fImaxvalue\fR
.br
NO MAXVALUE
.RS 4
The optional clause
MAXVALUE \fImaxvalue\fR
determines the maximum value for the sequence\&. If this clause is not supplied or
\fBNO MAXVALUE\fR
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\&.
.RE
.PP
\fIstart\fR
.RS 4
The optional clause
START WITH \fIstart\fR
allows the sequence to begin anywhere\&. The default starting value is
\fIminvalue\fR
for ascending sequences and
\fImaxvalue\fR
for descending ones\&.
.RE
.PP
\fIcache\fR
.RS 4
The optional clause
CACHE \fIcache\fR
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\&.
.RE
.PP
CYCLE
.br
NO CYCLE
.RS 4
The
CYCLE
option allows the sequence to wrap around when the
\fImaxvalue\fR
or
\fIminvalue\fR
has been reached by an ascending or descending sequence respectively\&. If the limit is reached, the next number generated will be the
\fIminvalue\fR
or
\fImaxvalue\fR, respectively\&.
.sp
If
NO CYCLE
is specified, any calls to
\fBnextval\fR
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\&.
.RE
.PP
OWNED BY \fItable_name\fR\&.\fIcolumn_name\fR
.br
OWNED BY NONE
.RS 4
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\&.
.RE
.SH "NOTES"
.PP
Use
\fBDROP SEQUENCE\fR
to remove a sequence\&.
.PP
Sequences are based on
bigint
arithmetic, so the range cannot exceed the range of an eight\-byte integer (\-9223372036854775808 to 9223372036854775807)\&.
.PP
Because
\fBnextval\fR
and
\fBsetval\fR
calls are never rolled back, sequence objects cannot be used if
\(lqgapless\(rq
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\&.
.PP
Unexpected results might be obtained if a
\fIcache\fR
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\*(Aqs
last_value
accordingly\&. Then, the next
\fIcache\fR\-1 uses of
\fBnextval\fR
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
\(lqholes\(rq
in the sequence\&.
.PP
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
\fIcache\fR
setting of 10, session A might reserve values 1\&.\&.10 and return
\fBnextval\fR=1, then session B might reserve values 11\&.\&.20 and return
\fBnextval\fR=11 before session A has generated
\fBnextval\fR=2\&. Thus, with a
\fIcache\fR
setting of one it is safe to assume that
\fBnextval\fR
values are generated sequentially; with a
\fIcache\fR
setting greater than one you should only assume that the
\fBnextval\fR
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
\fBnextval\fR\&.
.PP
Another consideration is that a
\fBsetval\fR
executed on such a sequence will not be noticed by other sessions until they have used up any preallocated values they have cached\&.
.SH "EXAMPLES"
.PP
Create an ascending sequence called
serial, starting at 101:
.sp
.if n \{\
.RS 4
.\}
.nf
CREATE SEQUENCE serial START 101;
.fi
.if n \{\
.RE
.\}
.PP
Select the next number from this sequence:
.sp
.if n \{\
.RS 4
.\}
.nf
SELECT nextval(\*(Aqserial\*(Aq);

 nextval
\-\-\-\-\-\-\-\-\-
     101
.fi
.if n \{\
.RE
.\}
.PP
Select the next number from this sequence:
.sp
.if n \{\
.RS 4
.\}
.nf
SELECT nextval(\*(Aqserial\*(Aq);

 nextval
\-\-\-\-\-\-\-\-\-
     102
.fi
.if n \{\
.RE
.\}
.PP
Use this sequence in an
\fBINSERT\fR
command:
.sp
.if n \{\
.RS 4
.\}
.nf
INSERT INTO distributors VALUES (nextval(\*(Aqserial\*(Aq), \*(Aqnothing\*(Aq);
.fi
.if n \{\
.RE
.\}
.PP
Update the sequence value after a
\fBCOPY FROM\fR:
.sp
.if n \{\
.RS 4
.\}
.nf
BEGIN;
COPY distributors FROM \*(Aqinput_file\*(Aq;
SELECT setval(\*(Aqserial\*(Aq, max(id)) FROM distributors;
END;
.fi
.if n \{\
.RE
.\}
.SH "COMPATIBILITY"
.PP
\fBCREATE SEQUENCE\fR
conforms to the
SQL
standard, with the following exceptions:
.sp
.RS 4
.ie n \{\
\h'-04'\(bu\h'+03'\c
.\}
.el \{\
.sp -1
.IP \(bu 2.3
.\}
Obtaining the next value is done using the
\fBnextval()\fR
function instead of the standard\*(Aqs
\fBNEXT VALUE FOR\fR
expression\&.
.RE
.sp
.RS 4
.ie n \{\
\h'-04'\(bu\h'+03'\c
.\}
.el \{\
.sp -1
.IP \(bu 2.3
.\}
The
OWNED BY
clause is a
PostgreSQL
extension\&.
.RE
.SH "SEE ALSO"
ALTER SEQUENCE (\fBALTER_SEQUENCE\fR(7)), DROP SEQUENCE (\fBDROP_SEQUENCE\fR(7))