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postgres/doc/src/sgml/ref/create_sequence.sgml
Peter Eisentraut b6ea172ace Spell checking and markup additions
2002-03-22 19:20:45 +00:00

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<!--
$Header: /cvsroot/pgsql/doc/src/sgml/ref/create_sequence.sgml,v 1.25 2002/03/22 19:20:40 petere Exp $
PostgreSQL documentation
-->
<refentry id="SQL-CREATESEQUENCE">
<refmeta>
<refentrytitle id="sql-createsequence-title">
CREATE SEQUENCE
</refentrytitle>
<refmiscinfo>SQL - Language Statements</refmiscinfo>
</refmeta>
<refnamediv>
<refname>
CREATE SEQUENCE
</refname>
<refpurpose>
define a new sequence generator
</refpurpose>
</refnamediv>
<refsynopsisdiv>
<refsynopsisdivinfo>
<date>1999-07-20</date>
</refsynopsisdivinfo>
<synopsis>
CREATE [ TEMPORARY | TEMP ] SEQUENCE <replaceable class="parameter">seqname</replaceable> [ INCREMENT <replaceable class="parameter">increment</replaceable> ]
[ MINVALUE <replaceable class="parameter">minvalue</replaceable> ] [ MAXVALUE <replaceable class="parameter">maxvalue</replaceable> ]
[ START <replaceable class="parameter">start</replaceable> ] [ CACHE <replaceable class="parameter">cache</replaceable> ] [ CYCLE ]
</synopsis>
<refsect2 id="R2-SQL-CREATESEQUENCE-1">
<refsect2info>
<date>1998-09-11</date>
</refsect2info>
<title>
Inputs
</title>
<para>
<variablelist>
<varlistentry>
<term>TEMPORARY or TEMP</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.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><replaceable class="parameter">seqname</replaceable></term>
<listitem>
<para>
The name of a sequence to be created.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><replaceable class="parameter">increment</replaceable></term>
<listitem>
<para>
The
<option>INCREMENT <replaceable class="parameter">increment</replaceable></option>
clause is optional. A positive value will make an
ascending sequence, a negative one a descending sequence.
The default value is one (1).
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><replaceable class="parameter">minvalue</replaceable></term>
<listitem>
<para>
The optional clause <option>MINVALUE
<replaceable class="parameter">minvalue</replaceable></option>
determines the minimum value
a sequence can generate. The defaults are 1 and -2^63-1 for
ascending and descending sequences, respectively.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><replaceable class="parameter">maxvalue</replaceable></term>
<listitem>
<para>
The optional clause <option>MAXVALUE
<replaceable class="parameter">maxvalue</replaceable></option>
determines the maximum
value for the sequence. The defaults are 2^63-1 and -1 for
ascending and descending sequences, respectively.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><replaceable class="parameter">start</replaceable></term>
<listitem>
<para>
The optional <option>START
<replaceable class="parameter">start</replaceable>
clause</option> enables 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 <option>CACHE <replaceable class="parameter">cache</replaceable></option> 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.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term>CYCLE</term>
<listitem>
<para>
The optional CYCLE keyword may be used to enable 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.
Without CYCLE, after the limit is reached <function>nextval</> calls
will return an error.
</para>
</listitem>
</varlistentry>
</variablelist>
</para>
</refsect2>
<refsect2 id="R2-SQL-CREATESEQUENCE-2">
<refsect2info>
<date>1998-09-11</date>
</refsect2info>
<title>
Outputs
</title>
<para>
<variablelist>
<varlistentry>
<term><computeroutput>
CREATE
</computeroutput></term>
<listitem>
<para>
Message returned if the command is successful.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><computeroutput>
ERROR: Relation '<replaceable class="parameter">seqname</replaceable>' already exists
</computeroutput></term>
<listitem>
<para>
If the sequence specified already exists.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><computeroutput>
ERROR: DefineSequence: MINVALUE (<replaceable class="parameter">start</replaceable>) can't be >= MAXVALUE (<replaceable class="parameter">max</replaceable>)
</computeroutput></term>
<listitem>
<para>
If the specified starting value is out of range.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><computeroutput>
ERROR: DefineSequence: START value (<replaceable class="parameter">start</replaceable>) can't be < MINVALUE (<replaceable class="parameter">min</replaceable>)
</computeroutput></term>
<listitem>
<para>
If the specified starting value is out of range.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><computeroutput>
ERROR: DefineSequence: MINVALUE (<replaceable class="parameter">min</replaceable>) can't be >= MAXVALUE (<replaceable class="parameter">max</replaceable>)
</computeroutput></term>
<listitem>
<para>
If the minimum and maximum values are inconsistent.
</para>
</listitem>
</varlistentry>
</variablelist>
</para>
</refsect2>
</refsynopsisdiv>
<refsect1 id="R1-SQL-CREATESEQUENCE-1">
<refsect1info>
<date>1998-09-11</date>
</refsect1info>
<title>
Description
</title>
<para>
<command>CREATE SEQUENCE</command> will enter a new sequence number generator
into the current database. This involves creating and initializing a
new single-row
table with the name <replaceable class="parameter">seqname</replaceable>.
The generator will be owned by the user issuing the command.
</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
the <citetitle>User's Guide</citetitle>.
</para>
<para>
Although you cannot update a sequence directly, you can use a query like
<programlisting>
SELECT * FROM <replaceable>seqname</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 backend process. (Of course, this value may be obsolete
by the time it's printed, if other processes are actively doing
<function>nextval</> calls.)
</para>
<caution>
<para>
Unexpected results may 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
backends. Each backend 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 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 <replaceable class="parameter">cache</replaceable>
setting of 10, backend A might reserve values 1..10 and return <function>nextval</function>=1,
then
backend B might reserve values 11..20 and return <function>nextval</function>=11 before backend
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 backend,
whether or not it has yet been returned by <function>nextval</>.
Another consideration is that a <function>setval</> executed on such a sequence
will not be noticed by other backends until they have used up any
preallocated values they have cached.
</para>
</caution>
<refsect2 id="R2-SQL-CREATESEQUENCE-3">
<refsect2info>
<date>1998-09-11</date>
</refsect2info>
<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 may be no compiler support for eight-byte integers, in which case
sequences use regular <type>integer</> arithmetic (range
-2147483648 to +2147483647).
</para>
<para>
When <replaceable class="parameter">cache</replaceable> 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 <quote>holes</quote> in the sequence.
</para>
</refsect2>
</refsect1>
<refsect1 id="R1-SQL-CREATESEQUENCE-2">
<title>
Usage
</title>
<para>
Create an ascending sequence called <literal>serial</literal>, starting at 101:
</para>
<programlisting>
CREATE SEQUENCE serial START 101;
</programlisting>
<para>
Select the next number from this sequence:
<programlisting>
SELECT nextval('serial');
nextval
-------
114
</programlisting>
</para>
<para>
Use this sequence in an INSERT:
<programlisting>
INSERT INTO distributors VALUES (nextval('serial'), 'nothing');
</programlisting>
</para>
<para>
Update the sequence value after a COPY FROM:
<programlisting>
BEGIN;
COPY distributors FROM 'input_file';
SELECT setval('serial', max(id)) FROM distributors;
END;
</programlisting>
</para>
</refsect1>
<refsect1 id="R1-SQL-CREATESEQUENCE-3">
<title>
Compatibility
</title>
<refsect2 id="R2-SQL-CREATESEQUENCE-4">
<refsect2info>
<date>1998-09-11</date>
</refsect2info>
<title>
SQL92
</title>
<para>
<command>CREATE SEQUENCE</command> is a <productname>PostgreSQL</productname>
language extension.
There is no <command>CREATE SEQUENCE</command> statement
in <acronym>SQL92</acronym>.
</para>
</refsect2>
</refsect1>
</refentry>
<!-- Keep this comment at the end of the file
Local variables:
mode: sgml
sgml-omittag:nil
sgml-shorttag:t
sgml-minimize-attributes:nil
sgml-always-quote-attributes:t
sgml-indent-step:1
sgml-indent-data:t
sgml-parent-document:nil
sgml-default-dtd-file:"../reference.ced"
sgml-exposed-tags:nil
sgml-local-catalogs:"/usr/lib/sgml/catalog"
sgml-local-ecat-files:nil
End:
-->
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