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$PostgreSQL: pgsql/doc/src/sgml/ref/prepare.sgml,v 1.19 2006/02/26 03:20:46 neilc Exp $
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<refentry id="SQL-PREPARE">
 <refmeta>
  <refentrytitle id="sql-prepare-title">PREPARE</refentrytitle>
  <refmiscinfo>SQL - Language Statements</refmiscinfo>
 </refmeta>

 <refnamediv>
  <refname>PREPARE</refname>
  <refpurpose>prepare a statement for execution</refpurpose>
 </refnamediv>

 <indexterm zone="sql-prepare">
  <primary>PREPARE</primary>
 </indexterm>

 <indexterm zone="sql-prepare">
  <primary>prepared statements</primary>
  <secondary>creating</secondary>
 </indexterm>

 <refsynopsisdiv>
<synopsis>
PREPARE <replaceable class="PARAMETER">name</replaceable> [ (<replaceable class="PARAMETER">datatype</replaceable> [, ...] ) ] AS <replaceable class="PARAMETER">statement</replaceable>
</synopsis>
 </refsynopsisdiv>

 <refsect1>
  <title>Description</title>

  <para>
   <command>PREPARE</command> creates a prepared statement. A prepared
   statement is a server-side object that can be used to optimize
   performance. When the <command>PREPARE</command> statement is
   executed, the specified statement is parsed, rewritten, and
   planned. When an <command>EXECUTE</command> command is subsequently
   issued, the prepared statement need only be executed. Thus, the
   parsing, rewriting, and planning stages are only performed once,
   instead of every time the statement is executed.
  </para>

  <para>
   Prepared statements can take parameters: values that are
   substituted into the statement when it is executed. When creating
   the prepared statement, refer to parameters by position, using
   <literal>$1</>, <literal>$2</>, etc. A corresponding list of
   parameter data types can optionally be specified. When a
   parameter's data type is not specified or is declared as
   <literal>unknown</literal>, the type is inferred from the context
   in which the parameter is used (if possible). When executing the
   statement, specify the actual values for these parameters in the
   <command>EXECUTE</command> statement.  Refer to <xref
   linkend="sql-execute" endterm="sql-execute-title"> for more
   information about that.
  </para>

  <para>
   Prepared statements only last for the duration of the current
   database session. When the session ends, the prepared statement is
   forgotten, so it must be recreated before being used again. This 
   also means that a single  prepared statement cannot be used by
   multiple simultaneous database clients; however, each client can create
   their own prepared statement to use.  The prepared statement can be
   manually cleaned up using the <xref linkend="sql-deallocate"
   endterm="sql-deallocate-title"> command.
  </para>

  <para>
   Prepared statements have the largest performance advantage when a
   single session is being used to execute a large number of similar
   statements. The performance difference will be particularly
   significant if the statements are complex to plan or rewrite, for
   example, if the query involves a join of many tables or requires
   the application of several rules. If the statement is relatively simple
   to plan and rewrite but relatively expensive to execute, the
   performance advantage of prepared statements will be less noticeable.
  </para>
 </refsect1>

 <refsect1>
  <title>Parameters</title>

  <variablelist>
   <varlistentry>
    <term><replaceable class="PARAMETER">name</replaceable></term>
    <listitem>
     <para>
      An arbitrary name given to this particular prepared
      statement. It must be unique within a single session and is
      subsequently used to execute or deallocate a previously prepared
      statement.
     </para>
    </listitem>
   </varlistentry>

   <varlistentry>
    <term><replaceable class="PARAMETER">datatype</replaceable></term>
    <listitem>
     <para>
      The data type of a parameter to the prepared statement.  If the
      data type of a particular parameter is unspecified or is
      specified as <literal>unknown</literal>, it will be inferred
      from the context in which the parameter is used. To refer to the
      parameters in the prepared statement itself, use
      <literal>$1</literal>, <literal>$2</literal>, etc.
     </para>
    </listitem>
   </varlistentry>

   <varlistentry>
    <term><replaceable class="PARAMETER">statement</replaceable></term>
    <listitem>
     <para>
      Any <command>SELECT</>, <command>INSERT</>, <command>UPDATE</>,
      or <command>DELETE</> statement.
     </para>
    </listitem>
   </varlistentry>
  </variablelist>
 </refsect1>

 <refsect1>
  <title>Notes</title>

  <para>
   In some situations, the query plan produced for a prepared
   statement will be inferior to the query plan that would have been
   chosen if the statement had been submitted and executed
   normally. This is because when the statement is planned and the
   planner attempts to determine the optimal query plan, the actual
   values of any parameters specified in the statement are
   unavailable. <productname>PostgreSQL</productname> collects
   statistics on the distribution of data in the table, and can use
   constant values in a statement to make guesses about the likely
   result of executing the statement. Since this data is unavailable
   when planning prepared statements with parameters, the chosen plan
   may be suboptimal. To examine the query plan
   <productname>PostgreSQL</productname> has chosen for a prepared
   statement, use <xref linkend="sql-explain"
   endterm="sql-explain-title">.
  </para>

  <para>
   For more information on query planning and the statistics collected
   by <productname>PostgreSQL</productname> for that purpose, see
   the <xref linkend="sql-analyze" endterm="sql-analyze-title">
   documentation.
  </para>

  <para>
   You can see all available prepared statements of a session by querying the
   <link linkend="view-pg-prepared-statements"><structname>pg_prepared_statements</structname></link>
   system view.
  </para>
 </refsect1>

 <refsect1 id="sql-prepare-examples">
  <title id="sql-prepare-examples-title">Examples</title>
  <para>
   Create a prepared statement for an <command>INSERT</command>
   statement, and then execute it:
<programlisting>
PREPARE fooplan (int, text, bool, numeric) AS
    INSERT INTO foo VALUES($1, $2, $3, $4);
EXECUTE fooplan(1, 'Hunter Valley', 't', 200.00);
</programlisting>
  </para>

  <para>
   Create a prepared statement for a <command>SELECT</command>
   statement, and then execute it:
<programlisting>
PREPARE usrrptplan (int) AS
    SELECT * FROM users u, logs l WHERE u.usrid=$1 AND u.usrid=l.usrid
    AND l.date = $2;
EXECUTE usrrptplan(1, current_date);
</programlisting>

   Note that the data type of the second parameter is not specified,
   so it is inferred from the context in which <literal>$2</> is used.
  </para>
 </refsect1>
 <refsect1>
  <title>Compatibility</title>

  <para>
   The SQL standard includes a <command>PREPARE</command> statement,
   but it is only for use in embedded SQL. This version of the
   <command>PREPARE</command> statement also uses a somewhat different
   syntax.
  </para>
 </refsect1>

 <refsect1>
  <title>See Also</title>

  <simplelist type="inline">
   <member><xref linkend="sql-deallocate" endterm="sql-deallocate-title"></member>
   <member><xref linkend="sql-execute" endterm="sql-execute-title"></member>
  </simplelist>
 </refsect1>
</refentry>

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