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$Header: /cvsroot/pgsql/doc/src/sgml/ref/alter_sequence.sgml,v 1.5 2003/09/22 00:16:57 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
	<option>INCREMENT BY <replaceable class="parameter">increment</replaceable></option>
	clause 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>NO MINVALUE</term>
      <listitem>
       <para>
	The optional clause <option>MINVALUE
	 <replaceable class="parameter">minvalue</replaceable></option>
	determines the minimum value
	a sequence can generate. If <option>NO MINVALUE</option> is specified,
    the defaults of 1 and -2^63-1 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>NO MAXVALUE</term>
      <listitem>
       <para>
	The optional clause <option>MAXVALUE
	 <replaceable class="parameter">maxvalue</replaceable></option>
	determines the maximum value for the sequence. If 
    <option>NO MAXVALUE</option> is specified, the defaults are 2^63-1 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 <option>RESTART WITH
	 <replaceable class="parameter">start</replaceable></option>
	 clause changes the current value of the sequence.
       </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).
	If unspecified, the old cache value will be maintained.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term>CYCLE</term>
      <listitem>
       <para>
	The optional <option>CYCLE</option> 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.
       </para>
      </listitem>
     </varlistentry>

	 <varlistentry>
	  <term>NO CYCLE</term>
	  <listitem>
	   <para>
	If the optional <option>NO CYCLE</option> keyword is specified, any
	calls to <function>nextval</function> after the sequence has reached
	its maximum value will return an error.  If neither
	<option>CYCLE</option> or <option>NO CYCLE</option> 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:
  </para>
  <programlisting>
ALTER SEQUENCE serial RESTART WITH 105;
  </programlisting>
 </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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