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<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd"> <HTML ><HEAD ><TITLE >PREPARE</TITLE ><META NAME="GENERATOR" CONTENT="Modular DocBook HTML Stylesheet Version 1.79"><LINK REV="MADE" HREF="mailto:pgsql-docs@postgresql.org"><LINK REL="HOME" TITLE="PostgreSQL 9.2.24 Documentation" HREF="index.html"><LINK REL="UP" TITLE="SQL Commands" HREF="sql-commands.html"><LINK REL="PREVIOUS" TITLE="NOTIFY" HREF="sql-notify.html"><LINK REL="NEXT" TITLE="PREPARE TRANSACTION" HREF="sql-prepare-transaction.html"><LINK REL="STYLESHEET" TYPE="text/css" HREF="stylesheet.css"><META HTTP-EQUIV="Content-Type" CONTENT="text/html; charset=ISO-8859-1"><META NAME="creation" CONTENT="2017-11-06T22:43:11"></HEAD ><BODY CLASS="REFENTRY" ><DIV CLASS="NAVHEADER" ><TABLE SUMMARY="Header navigation table" WIDTH="100%" BORDER="0" CELLPADDING="0" CELLSPACING="0" ><TR ><TH COLSPAN="5" ALIGN="center" VALIGN="bottom" ><A HREF="index.html" >PostgreSQL 9.2.24 Documentation</A ></TH ></TR ><TR ><TD WIDTH="10%" ALIGN="left" VALIGN="top" ><A TITLE="NOTIFY" HREF="sql-notify.html" ACCESSKEY="P" >Prev</A ></TD ><TD WIDTH="10%" ALIGN="left" VALIGN="top" ><A HREF="sql-commands.html" ACCESSKEY="U" >Up</A ></TD ><TD WIDTH="60%" ALIGN="center" VALIGN="bottom" ></TD ><TD WIDTH="20%" ALIGN="right" VALIGN="top" ><A TITLE="PREPARE TRANSACTION" HREF="sql-prepare-transaction.html" ACCESSKEY="N" >Next</A ></TD ></TR ></TABLE ><HR ALIGN="LEFT" WIDTH="100%"></DIV ><H1 ><A NAME="SQL-PREPARE" ></A >PREPARE</H1 ><DIV CLASS="REFNAMEDIV" ><A NAME="AEN76601" ></A ><H2 >Name</H2 >PREPARE -- prepare a statement for execution</DIV ><DIV CLASS="REFSYNOPSISDIV" ><A NAME="AEN76609" ></A ><H2 >Synopsis</H2 ><PRE CLASS="SYNOPSIS" >PREPARE <TT CLASS="REPLACEABLE" ><I >name</I ></TT > [ ( <TT CLASS="REPLACEABLE" ><I >data_type</I ></TT > [, ...] ) ] AS <TT CLASS="REPLACEABLE" ><I >statement</I ></TT ></PRE ></DIV ><DIV CLASS="REFSECT1" ><A NAME="AEN76614" ></A ><H2 >Description</H2 ><P > <TT CLASS="COMMAND" >PREPARE</TT > creates a prepared statement. A prepared statement is a server-side object that can be used to optimize performance. When the <TT CLASS="COMMAND" >PREPARE</TT > statement is executed, the specified statement is parsed, analyzed, and rewritten. When an <TT CLASS="COMMAND" >EXECUTE</TT > 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. </P ><P > 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 <TT CLASS="LITERAL" >$1</TT >, <TT CLASS="LITERAL" >$2</TT >, 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 <TT CLASS="LITERAL" >unknown</TT >, 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 <TT CLASS="COMMAND" >EXECUTE</TT > statement. Refer to <A HREF="sql-execute.html" >EXECUTE</A > for more information about that. </P ><P > 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 <A HREF="sql-deallocate.html" >DEALLOCATE</A > command. </P ><P > 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. </P ></DIV ><DIV CLASS="REFSECT1" ><A NAME="AEN76629" ></A ><H2 >Parameters</H2 ><P ></P ><DIV CLASS="VARIABLELIST" ><DL ><DT ><TT CLASS="REPLACEABLE" ><I >name</I ></TT ></DT ><DD ><P > 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. </P ></DD ><DT ><TT CLASS="REPLACEABLE" ><I >data_type</I ></TT ></DT ><DD ><P > The data type of a parameter to the prepared statement. If the data type of a particular parameter is unspecified or is specified as <TT CLASS="LITERAL" >unknown</TT >, it will be inferred from the context in which the parameter is used. To refer to the parameters in the prepared statement itself, use <TT CLASS="LITERAL" >$1</TT >, <TT CLASS="LITERAL" >$2</TT >, etc. </P ></DD ><DT ><TT CLASS="REPLACEABLE" ><I >statement</I ></TT ></DT ><DD ><P > Any <TT CLASS="COMMAND" >SELECT</TT >, <TT CLASS="COMMAND" >INSERT</TT >, <TT CLASS="COMMAND" >UPDATE</TT >, <TT CLASS="COMMAND" >DELETE</TT >, or <TT CLASS="COMMAND" >VALUES</TT > statement. </P ></DD ></DL ></DIV ></DIV ><DIV CLASS="REFSECT1" ><A NAME="AEN76655" ></A ><H2 >Notes</H2 ><P > If a prepared statement is executed enough times, the server may eventually decide to save and re-use a generic plan rather than re-planning each time. This will occur immediately if the prepared statement has no parameters; otherwise it occurs only if the generic plan appears to be not much more expensive than a plan that depends on specific parameter values. Typically, a generic plan will be selected only if the query's performance is estimated to be fairly insensitive to the specific parameter values supplied. </P ><P > To examine the query plan <SPAN CLASS="PRODUCTNAME" >PostgreSQL</SPAN > is using for a prepared statement, use <A HREF="sql-explain.html" >EXPLAIN</A >. If a generic plan is in use, it will contain parameter symbols <TT CLASS="LITERAL" >$<TT CLASS="REPLACEABLE" ><I >n</I ></TT ></TT >, while a custom plan will have the current actual parameter values substituted into it. </P ><P > For more information on query planning and the statistics collected by <SPAN CLASS="PRODUCTNAME" >PostgreSQL</SPAN > for that purpose, see the <A HREF="sql-analyze.html" >ANALYZE</A > documentation. </P ><P > You can see all prepared statements available in the session by querying the <A HREF="view-pg-prepared-statements.html" ><TT CLASS="STRUCTNAME" >pg_prepared_statements</TT ></A > system view. </P ></DIV ><DIV CLASS="REFSECT1" ><A NAME="SQL-PREPARE-EXAMPLES" ></A ><H2 >Examples</H2 ><P > Create a prepared statement for an <TT CLASS="COMMAND" >INSERT</TT > statement, and then execute it: </P><PRE CLASS="PROGRAMLISTING" >PREPARE fooplan (int, text, bool, numeric) AS INSERT INTO foo VALUES($1, $2, $3, $4); EXECUTE fooplan(1, 'Hunter Valley', 't', 200.00);</PRE ><P> </P ><P > Create a prepared statement for a <TT CLASS="COMMAND" >SELECT</TT > statement, and then execute it: </P><PRE CLASS="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);</PRE ><P> Note that the data type of the second parameter is not specified, so it is inferred from the context in which <TT CLASS="LITERAL" >$2</TT > is used. </P ></DIV ><DIV CLASS="REFSECT1" ><A NAME="AEN76678" ></A ><H2 >Compatibility</H2 ><P > The SQL standard includes a <TT CLASS="COMMAND" >PREPARE</TT > statement, but it is only for use in embedded SQL. This version of the <TT CLASS="COMMAND" >PREPARE</TT > statement also uses a somewhat different syntax. </P ></DIV ><DIV CLASS="REFSECT1" ><A NAME="AEN76683" ></A ><H2 >See Also</H2 ><A HREF="sql-deallocate.html" >DEALLOCATE</A >, <A HREF="sql-execute.html" >EXECUTE</A ></DIV ><DIV CLASS="NAVFOOTER" ><HR ALIGN="LEFT" WIDTH="100%"><TABLE SUMMARY="Footer navigation table" WIDTH="100%" BORDER="0" CELLPADDING="0" CELLSPACING="0" ><TR ><TD WIDTH="33%" ALIGN="left" VALIGN="top" ><A HREF="sql-notify.html" ACCESSKEY="P" >Prev</A ></TD ><TD WIDTH="34%" ALIGN="center" VALIGN="top" ><A HREF="index.html" ACCESSKEY="H" >Home</A ></TD ><TD WIDTH="33%" ALIGN="right" VALIGN="top" ><A HREF="sql-prepare-transaction.html" ACCESSKEY="N" >Next</A ></TD ></TR ><TR ><TD WIDTH="33%" ALIGN="left" VALIGN="top" >NOTIFY</TD ><TD WIDTH="34%" ALIGN="center" VALIGN="top" ><A HREF="sql-commands.html" ACCESSKEY="U" >Up</A ></TD ><TD WIDTH="33%" ALIGN="right" VALIGN="top" >PREPARE TRANSACTION</TD ></TR ></TABLE ></DIV ></BODY ></HTML >