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

</synopsis>

</refsynopsisdiv>

<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"> 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"> 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>

<title>Parameters</title>

<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>

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<para>

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The data type of a parameter to the prepared statement. If the

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data type of a particular parameter is unspecified or is

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specified as <literal>unknown</literal>, it will be inferred

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from the context in which the parameter is used. To refer to the

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parameters in the prepared statement itself, use

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<literal>$1</literal>, <literal>$2</literal>, etc.

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</para>

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</listitem>

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</varlistentry>

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<term><replaceable class="PARAMETER">statement</replaceable></term>

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<para>

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Any <command>SELECT</>, <command>INSERT</>, <command>UPDATE</>,

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<command>DELETE</>, or <command>VALUES</> statement.

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</para>

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</listitem>

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</varlistentry>

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</variablelist>

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</refsect1>

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<title>Notes</title>

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<para>

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In some situations, the query plan produced for a prepared

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statement will be inferior to the query plan that would have been

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chosen if the statement had been submitted and executed

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normally. This is because when the statement is planned and the

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planner attempts to determine the optimal query plan, the actual

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values of any parameters specified in the statement are

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unavailable. <productname>PostgreSQL</productname> collects

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statistics on the distribution of data in the table, and can use

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constant values in a statement to make guesses about the likely

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result of executing the statement. Since this data is unavailable

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when planning prepared statements with parameters, the chosen plan

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might be suboptimal. To examine the query plan

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<productname>PostgreSQL</productname> has chosen for a prepared

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statement, use <xref linkend="sql-explain">.

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</para>

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<para>

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For more information on query planning and the statistics collected

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by <productname>PostgreSQL</productname> for that purpose, see

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the <xref linkend="sql-analyze">

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documentation.

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</para>

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<para>

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You can see all available prepared statements of a session by querying the

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system view.

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</para>

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</refsect1>

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<title id="sql-prepare-examples-title">Examples</title>

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<para>

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Create a prepared statement for an <command>INSERT</command>

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statement, and then execute it:

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PREPARE fooplan (int, text, bool, numeric) AS

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INSERT INTO foo VALUES($1, $2, $3, $4);

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EXECUTE fooplan(1, 'Hunter Valley', 't', 200.00);

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</programlisting>

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</para>

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<para>

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Create a prepared statement for a <command>SELECT</command>

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statement, and then execute it:

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PREPARE usrrptplan (int) AS

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SELECT * FROM users u, logs l WHERE u.usrid=$1 AND u.usrid=l.usrid

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AND l.date = $2;

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EXECUTE usrrptplan(1, current_date);

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</programlisting>

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Note that the data type of the second parameter is not specified,

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so it is inferred from the context in which <literal>$2</> is used.

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</para>

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</refsect1>

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<title>Compatibility</title>

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<para>

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The SQL standard includes a <command>PREPARE</command> statement,

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but it is only for use in embedded SQL. This version of the

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<command>PREPARE</command> statement also uses a somewhat different

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syntax.

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</para>

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</refsect1>

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</simplelist>

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</refsect1>

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</refentry>

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