<!--
doc/src/sgml/ref/create_sequence.sgml
PostgreSQL documentation
-->
<refentry id="SQL-CREATESEQUENCE">
<refmeta>
<refentrytitle>CREATE SEQUENCE</refentrytitle>
<manvolnum>7</manvolnum>
<refmiscinfo>SQL - Language Statements</refmiscinfo>
</refmeta>
<refnamediv>
<refname>CREATE SEQUENCE</refname>
<refpurpose>define a new sequence generator</refpurpose>
</refnamediv>
<indexterm zone="sql-createsequence">
<primary>CREATE SEQUENCE</primary>
</indexterm>
<refsynopsisdiv>
<synopsis>
CREATE [ TEMPORARY | TEMP ] SEQUENCE <replaceable class="parameter">name</replaceable> [ INCREMENT [ BY ] <replaceable class="parameter">increment</replaceable> ]
[ MINVALUE <replaceable class="parameter">minvalue</replaceable> | NO MINVALUE ] [ MAXVALUE <replaceable class="parameter">maxvalue</replaceable> | NO MAXVALUE ]
[ START [ WITH ] <replaceable class="parameter">start</replaceable> ] [ CACHE <replaceable class="parameter">cache</replaceable> ] [ [ NO ] CYCLE ]
[ OWNED BY { <replaceable class="parameter">table_name</replaceable>.<replaceable class="parameter">column_name</replaceable> | NONE } ]
</synopsis>
</refsynopsisdiv>
<refsect1>
<title>Description</title>
<para>
<command>CREATE SEQUENCE</command> creates a new sequence number
generator. This involves creating and initializing a new special
single-row table with the name <replaceable
class="parameter">name</replaceable>. The generator will be
owned by the user issuing the command.
</para>
<para>
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 sequence,
table, index, view, or foreign table in the same schema.
</para>
<para>
After a sequence is created, you use the functions
<function>nextval</function>,
<function>currval</function>, and
<function>setval</function>
to operate on the sequence. These functions are documented in
<xref linkend="functions-sequence">.
</para>
<para>
Although you cannot update a sequence directly, you can use a query like:
<programlisting>
SELECT * FROM <replaceable>name</replaceable>;
</programlisting>
to examine the parameters and current state of a sequence. In particular,
the <literal>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
<function>nextval</> calls.)
</para>
</refsect1>
<refsect1>
<title>Parameters</title>
<variablelist>
<varlistentry>
<term><literal>TEMPORARY</literal> or <literal>TEMP</literal></term>
<listitem>
<para>
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.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><replaceable class="parameter">name</replaceable></term>
<listitem>
<para>
The name (optionally schema-qualified) of the sequence to be created.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><replaceable class="parameter">increment</replaceable></term>
<listitem>
<para>
The optional clause <literal>INCREMENT BY <replaceable
class="parameter">increment</replaceable></literal> 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.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><replaceable class="parameter">minvalue</replaceable></term>
<term><literal>NO MINVALUE</literal></term>
<listitem>
<para>
The optional clause <literal>MINVALUE <replaceable
class="parameter">minvalue</replaceable></literal> determines
the minimum value a sequence can generate. If this clause is not
supplied or <option>NO MINVALUE</option> is specified, then
defaults will be used. The defaults are 1 and
-2<superscript>63</>-1 for ascending and descending sequences,
respectively.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><replaceable class="parameter">maxvalue</replaceable></term>
<term><literal>NO MAXVALUE</literal></term>
<listitem>
<para>
The optional clause <literal>MAXVALUE <replaceable
class="parameter">maxvalue</replaceable></literal> determines
the maximum value for the sequence. If this clause is not
supplied or <option>NO MAXVALUE</option> is specified, then
default values will be used. The defaults are
2<superscript>63</>-1 and -1 for ascending and descending
sequences, respectively.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><replaceable class="parameter">start</replaceable></term>
<listitem>
<para>
The optional clause <literal>START WITH <replaceable
class="parameter">start</replaceable> </literal> allows the
sequence to begin anywhere. The default starting value is
<replaceable class="parameter">minvalue</replaceable> for
ascending sequences and <replaceable
class="parameter">maxvalue</replaceable> for descending ones.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><replaceable class="parameter">cache</replaceable></term>
<listitem>
<para>
The optional clause <literal>CACHE <replaceable
class="parameter">cache</replaceable></literal> 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.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><literal>CYCLE</literal></term>
<term><literal>NO CYCLE</literal></term>
<listitem>
<para>
The <literal>CYCLE</literal> option allows the sequence to wrap
around when the <replaceable
class="parameter">maxvalue</replaceable> or <replaceable
class="parameter">minvalue</replaceable> has been reached by an
ascending or descending sequence respectively. If the limit is
reached, the next number generated will be the <replaceable
class="parameter">minvalue</replaceable> or <replaceable
class="parameter">maxvalue</replaceable>, respectively.
</para>
<para>
If <literal>NO CYCLE</literal> is specified, any calls to
<function>nextval</function> after the sequence has reached its
maximum value will return an error. If neither
<literal>CYCLE</literal> or <literal>NO CYCLE</literal> are
specified, <literal>NO CYCLE</literal> is the default.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><literal>OWNED BY</literal> <replaceable class="parameter">table_name</replaceable>.<replaceable class="parameter">column_name</replaceable></term>
<term><literal>OWNED BY NONE</literal></term>
<listitem>
<para>
The <literal>OWNED BY</literal> 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.
<literal>OWNED BY NONE</literal>, the default, specifies that there
is no such association.
</para>
</listitem>
</varlistentry>
</variablelist>
</refsect1>
<refsect1>
<title>Notes</title>
<para>
Use <command>DROP SEQUENCE</command> to remove a sequence.
</para>
<para>
Sequences are based on <type>bigint</> arithmetic, so the range
cannot exceed the range of an eight-byte integer
(-9223372036854775808 to 9223372036854775807). On some older
platforms, there might be no compiler support for eight-byte
integers, in which case sequences use regular <type>integer</>
arithmetic (range -2147483648 to +2147483647).
</para>
<para>
Unexpected results might be obtained if a <replaceable
class="parameter">cache</replaceable> 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 <literal>last_value</> accordingly.
Then, the next <replaceable class="parameter">cache</replaceable>-1
uses of <function>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 <quote>holes</quote> in the
sequence.
</para>
<para>
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 <replaceable class="parameter">cache</replaceable> setting of 10,
session A might reserve values 1..10 and return
<function>nextval</function>=1, then session B might reserve values
11..20 and return <function>nextval</function>=11 before session A
has generated <literal>nextval</literal>=2. Thus, with a
<replaceable class="parameter">cache</replaceable> setting of one
it is safe to assume that <function>nextval</> values are generated
sequentially; with a <replaceable
class="parameter">cache</replaceable> setting greater than one you
should only assume that the <function>nextval</> values are all
distinct, not that they are generated purely sequentially. Also,
<literal>last_value</> will reflect the latest value reserved by
any session, whether or not it has yet been returned by
<function>nextval</>.
</para>
<para>
Another consideration is that a <function>setval</> executed on
such a sequence will not be noticed by other sessions until they
have used up any preallocated values they have cached.
</para>
</refsect1>
<refsect1>
<title>Examples</title>
<para>
Create an ascending sequence called <literal>serial</literal>, starting at 101:
<programlisting>
CREATE SEQUENCE serial START 101;
</programlisting>
</para>
<para>
Select the next number from this sequence:
<programlisting>
SELECT nextval('serial');
nextval
---------
101
</programlisting>
</para>
<para>
Select the next number from this sequence:
<programlisting>
SELECT nextval('serial');
nextval
---------
102
</programlisting>
</para>
<para>
Use this sequence in an <command>INSERT</command> command:
<programlisting>
INSERT INTO distributors VALUES (nextval('serial'), 'nothing');
</programlisting>
</para>
<para>
Update the sequence value after a <command>COPY FROM</command>:
<programlisting>
BEGIN;
COPY distributors FROM 'input_file';
SELECT setval('serial', max(id)) FROM distributors;
END;
</programlisting></para>
</refsect1>
<refsect1>
<title>Compatibility</title>
<para>
<command>CREATE SEQUENCE</command> conforms to the <acronym>SQL</acronym>
standard, with the following exceptions:
<itemizedlist>
<listitem>
<para>
The standard's <literal>AS <data type></literal> expression is not
supported.
</para>
</listitem>
<listitem>
<para>
Obtaining the next value is done using the <function>nextval()</>
function instead of the standard's <command>NEXT VALUE FOR</command>
expression.
</para>
</listitem>
<listitem>
<para>
The <literal>OWNED BY</> clause is a <productname>PostgreSQL</>
extension.
</para>
</listitem>
</itemizedlist></para>
</refsect1>
<refsect1>
<title>See Also</title>
<simplelist type="inline">
<member><xref linkend="sql-altersequence"></member>
<member><xref linkend="sql-dropsequence"></member>
</simplelist>
</refsect1>
</refentry>