create_sequence.sgml

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$Header: /cvsroot/pgsql/doc/src/sgml/ref/create_sequence.sgml,v 1.28 2002/05/18 15:44:47 petere Exp $
PostgreSQL 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>
   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, unless
	they are referenced with schema-qualified names.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term><replaceable class="parameter">seqname</replaceable></term>
      <listitem>
       <para>
	The name (optionally schema-qualified) 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 SEQUENCE
       </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>
   If a schema name is given then the sequence is created in the
   specified schema.  Otherwise it is created in the current schema (the one
   at the front of the search path; see <literal>CURRENT_SCHEMA()</>).
   TEMP sequences exist in a special schema, so a schema name may not be
   given when creating a TEMP sequence.
   The sequence name must be distinct from the name of any other sequence,
   table, index, or view 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
   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>

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