create_sequence.sgml

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$Header: /cvsroot/pgsql/doc/src/sgml/ref/create_sequence.sgml,v 1.12 2000/05/08 16:19:56 thomas Exp $
Postgres documentation
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<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>
   Creates a new sequence number generator
  </refpurpose>
 </refnamediv> 
 <refsynopsisdiv>
  <refsynopsisdivinfo>
   <date>1999-07-20</date>
  </refsynopsisdivinfo>
  <synopsis>
CREATE 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><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 -2147483647 for
	ascending and descending sequences, respectively.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term><replaceable class="parameter">maxvalue</replaceable></term>
      <listitem>
       <para>
	Use the optional clause <option>MAXVALUE
	 <replaceable class="parameter">maxvalue</replaceable></option> to
	determine the maximum
	value for the sequence. The defaults are 2147483647 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 continue 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 whatever the
	<replaceable class="parameter">minvalue</replaceable> or
	<replaceable class="parameter">maxvalue</replaceable> is,
	as appropriate.
       </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 inconsistant.
       </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 data base. 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 may use the function
   <function>nextval(<replaceable class="parameter">seqname</replaceable>)</function>
   to get a new number from the sequence.
   The function
   <function>currval('<replaceable class="parameter">seqname</replaceable>')</function>
   may be used to determine the number returned by the last call to
   <function>nextval(<replaceable class="parameter">seqname</replaceable>)</function>
   for the specified sequence in the current session.
   The function
   <function>setval('<replaceable class="parameter">seqname</replaceable>',
    <replaceable class="parameter">newvalue</replaceable>)</function>
   may be used to set the current value of the specified sequence.
   The next call to 
   <function>nextval(<replaceable class="parameter">seqname</replaceable>)</function>
   will return the given value plus the sequence increment.
  </para>

  <para>
   Use a query like

   <programlisting>
SELECT * FROM <replaceable>seqname</replaceable>;
   </programlisting>

   to get the parameters of a sequence.

   As an alternative to fetching the
   parameters from the original definition as above, you can use

   <programlisting>
SELECT last_value FROM <replaceable>seqname</replaceable>;
   </programlisting>

   to obtain the last value allocated by any backend.
  </para>

  <para>
   Low-level locking is used to enable multiple simultaneous
   calls to a generator.
  </para>

  <caution>
   <para>
    Unexpected results may be obtained if a cache 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 last_value accordingly.  Then, the next cache-1 uses of nextval
    within that backend simply return the preallocated values without touching
    the shared object.  So, numbers allocated but not used in the current session
    will be lost.  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 cache
    setting of 10, backend A might reserve values 1..10 and return nextval=1, 
    then
    backend B might reserve values 11..20 and return nextval=11 before backend
    A has generated nextval=2.)  Thus, with a cache setting of one it is safe
    to assume that nextval values are generated sequentially; with a cache
    setting greater than one you should only assume that the nextval values
    are all distinct, not that they are generated purely sequentially.
    Also, last_value will reflect the latest value reserved by any backend,
    whether or not it has yet been returned by nextval.
   </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>
    Each backend uses its own cache to store allocated numbers.
    Numbers that are cached but not used in the current session will be
    lost, resulting in "holes" 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>
   Set the sequence value after a COPY FROM:
   <programlisting>
CREATE FUNCTION distributors_id_max() RETURNS INT4
    AS 'SELECT max(id) FROM distributors' 
    LANGUAGE 'sql';
BEGIN;
    COPY distributors FROM 'input_file';
    SELECT setval('serial', distributors_id_max());
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>Postgres</productname>
    language extension.
    There is no <command>CREATE SEQUENCE</command> statement
    in <acronym>SQL92</acronym>.
   </para>
  </refsect2>
 </refsect1>
</refentry>

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