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$Header: /cvsroot/pgsql/doc/src/sgml/ref/alter_sequence.sgml,v 1.6 2003/11/01 01:56:29 petere Exp $
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<refentry id="SQL-ALTERSEQUENCE">
 <refmeta>
  <refentrytitle id="SQL-ALTERSEQUENCE-TITLE">ALTER SEQUENCE</refentrytitle>
  <refmiscinfo>SQL - Language Statements</refmiscinfo>
 </refmeta>

 <refnamediv>
  <refname>
   ALTER SEQUENCE
  </refname>
  <refpurpose>
   alter the definition of a sequence generator
  </refpurpose>
 </refnamediv> 

 <indexterm zone="sql-altersequence">
  <primary>ALTER SEQUENCE</primary>
 </indexterm>

 <refsynopsisdiv>
  <synopsis>
ALTER 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 ]
    [ RESTART [ WITH ] <replaceable class="parameter">start</replaceable> ] [ CACHE <replaceable class="parameter">cache</replaceable> ] [ [ NO ] CYCLE ]
  </synopsis>
 </refsynopsisdiv>

 <refsect1>
  <title>Description</title>

  <para>
   <command>ALTER SEQUENCE</command> changes the parameters of an existing
   sequence generator.  Any parameter not specifically set in the
   <command>ALTER SEQUENCE</command> command retains its prior setting.
  </para>
 </refsect1>

 <refsect1>
  <title>Parameters</title>

   <para>
    <variablelist>
     <varlistentry>
      <term><replaceable class="parameter">name</replaceable></term>
      <listitem>
       <para>
        The name (optionally schema-qualified) of a sequence to be altered.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term><replaceable class="parameter">increment</replaceable></term>
      <listitem>
       <para>
        The clause <literal>INCREMENT BY <replaceable
        class="parameter">increment</replaceable></literal> is
        optional. A positive value will make an ascending sequence, a
        negative one a descending sequence.  If unspecified, the old
        increment value will be maintained.
       </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 <literal>NO
        MINVALUE</literal> is specified, the defaults of 1 and
        -2<superscript>63</>-1 for ascending and descending sequences,
        respectively, will be used.  If neither option is specified,
        the current minimum value will be maintained.
       </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 <literal>NO
        MAXVALUE</literal> is specified, the defaults are
        2<superscript>63</>-1 and -1 for ascending and descending
        sequences, respectively, will be used.  If neither option is
        specified, the current maximum value will be maintained.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term><replaceable class="parameter">start</replaceable></term>
      <listitem>
       <para>
        The optional clause <literal>RESTART WITH <replaceable
        class="parameter">start</replaceable></literal> changes the
        current value of the sequence.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term><replaceable class="parameter">cache</replaceable></term>
      <listitem>
       <para>
        The clause <literal>CACHE <replaceable
        class="parameter">cache</replaceable></literal> 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).  If unspecified, the old
        cache value will be maintained.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term>CYCLE</term>
      <listitem>
       <para>
        The optional <literal>CYCLE</literal> key word 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.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term>NO CYCLE</term>
      <listitem>
       <para>
        If the optional <literal>NO CYCLE</literal> key word 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, the old cycle behaviour will be
        maintained.
       </para>
      </listitem>
     </varlistentry>
    </variablelist>
   </para>
  </refsect1>

 <refsect1>
  <title>Examples</title>

  <para>
   Restart a sequence called <literal>serial</literal>, at 105:
<programlisting>
ALTER SEQUENCE serial RESTART WITH 105;
</programlisting>
  </para>
 </refsect1>

 <refsect1>
  <title>Notes</title>

  <para>
   To avoid blocking of concurrent transactions that obtain numbers from the
   same sequence, <command>ALTER SEQUENCE</command> is never rolled back;
   the changes take effect immediately and are not reversible.
  </para>

  <para>
   <command>ALTER SEQUENCE</command> will not immediately affect
   <literal>nextval</> results in backends,
   other than the current one, that have preallocated (cached) sequence
   values. They will use up all cached values prior to noticing the changed
   sequence parameters.  The current backend will be affected immediately.
  </para>
 </refsect1>


 <refsect1>
  <title>
   Compatibility
  </title>

  <refsect2>
   <title>
    SQL99
   </title>

   <para>
    <command>ALTER SEQUENCE</command> is a <productname>PostgreSQL</productname>
    language extension.
    There is no <command>ALTER SEQUENCE</command> statement
    in <acronym>SQL99</acronym>.
   </para>
  </refsect2>
 </refsect1>
</refentry>

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