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[postgresql] / doc / src / sgml / ref / prepare.sgml

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doc/src/sgml/ref/prepare.sgml
PostgreSQL documentation
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<refentry id="SQL-PREPARE">
 <indexterm zone="sql-prepare">
  <primary>PREPARE</primary>
 </indexterm>

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

 <refmeta>
  <refentrytitle>PREPARE</refentrytitle>
  <manvolnum>7</manvolnum>
  <refmiscinfo>SQL - Language Statements</refmiscinfo>
 </refmeta>

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

 <refsynopsisdiv>
<synopsis>
PREPARE <replaceable class="PARAMETER">name</replaceable> [ ( <replaceable class="PARAMETER">data_type</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, analyzed, and rewritten.
   When an <command>EXECUTE</command> command is subsequently
   issued, the prepared statement is planned and executed.  This division
   of labor avoids repetitive parse analysis work, while allowing
   the execution plan to depend on the specific parameter values supplied.
  </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"> 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.  Prepared statements can be
   manually cleaned up using the <xref linkend="sql-deallocate"> command.
  </para>

  <para>
   Prepared statements potentially 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, e.g.
   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">data_type</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</>,
      <command>DELETE</>, or <command>VALUES</> statement.
     </para>
    </listitem>
   </varlistentry>
  </variablelist>
 </refsect1>

 <refsect1 id="SQL-PREPARE-notes">
  <title>Notes</title>

  <para>
   Prepared statements can use generic plans rather than re-planning with
   each set of supplied <command>EXECUTE</command> values.  This occurs
   immediately for prepared statements with no parameters; otherwise
   it occurs only after five or more executions produce plans whose
   estimated cost average (including planning overhead) is more expensive
   than the generic plan cost estimate.  Once a generic plan is chosen,
   it is used for the remaining lifetime of the prepared statement.
   Using <command>EXECUTE</command> values which are rare in columns with
   many duplicates can generate custom plans that are so much cheaper
   than the generic plan, even after adding planning overhead, that the
   generic plan might never be used.
  </para>

  <para>
   A generic plan assumes that each value supplied to
   <command>EXECUTE</command> is one of the column's distinct values
   and that column values are uniformly distributed.  For example,
   if statistics record three distinct column values, a generic plan
   assumes a column equality comparison will match 33% of processed rows.
   Column statistics also allow generic plans to accurately compute the
   selectivity of unique columns.  Comparisons on non-uniformly-distributed
   columns and specification of non-existent values affects the average
   plan cost, and hence if and when a generic plan is chosen.
  </para>

  <para>
   To examine the query plan <productname>PostgreSQL</productname> is using
   for a prepared statement, use <xref linkend="sql-explain">, e.g.
   <command>EXPLAIN EXECUTE</>.
   If a generic plan is in use, it will contain parameter symbols
   <literal>$<replaceable>n</></literal>, while a custom plan will have the
   supplied parameter values substituted into it.
   The row estimates in the generic plan reflect the selectivity
   computed for the parameters.
  </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">
   documentation.
  </para>

  <para>
   Although the main point of a prepared statement is to avoid repeated parse
   analysis and planning of the statement, <productname>PostgreSQL</> will
   force re-analysis and re-planning of the statement before using it
   whenever database objects used in the statement have undergone
   definitional (DDL) changes since the previous use of the prepared
   statement.  Also, if the value of <xref linkend="guc-search-path"> changes
   from one use to the next, the statement will be re-parsed using the new
   <varname>search_path</>.  (This latter behavior is new as of
   <productname>PostgreSQL</productname> 9.3.)  These rules make use of a
   prepared statement semantically almost equivalent to re-submitting the
   same query text over and over, but with a performance benefit if no object
   definitions are changed, especially if the best plan remains the same
   across uses.  An example of a case where the semantic equivalence is not
   perfect is that if the statement refers to a table by an unqualified name,
   and then a new table of the same name is created in a schema appearing
   earlier in the <varname>search_path</>, no automatic re-parse will occur
   since no object used in the statement changed.  However, if some other
   change forces a re-parse, the new table will be referenced in subsequent
   uses.
  </para>

  <para>
   You can see all prepared statements available in the 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"></member>
   <member><xref linkend="sql-execute"></member>
  </simplelist>
 </refsect1>
</refentry>