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1<?xml version="1.0" encoding="UTF-8" standalone="no"?>2<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd"><html xmlns="http://www.w3.org/1999/xhtml"><head><meta http-equiv="Content-Type" content="text/html; charset=UTF-8" /><title>PREPARE</title><link rel="stylesheet" type="text/css" href="stylesheet.css" /><link rev="made" href="pgsql-docs@lists.postgresql.org" /><meta name="generator" content="DocBook XSL Stylesheets Vsnapshot" /><link rel="prev" href="sql-notify.html" title="NOTIFY" /><link rel="next" href="sql-prepare-transaction.html" title="PREPARE TRANSACTION" /></head><body id="docContent" class="container-fluid col-10"><div class="navheader"><table width="100%" summary="Navigation header"><tr><th colspan="5" align="center">PREPARE</th></tr><tr><td width="10%" align="left"><a accesskey="p" href="sql-notify.html" title="NOTIFY">Prev</a> </td><td width="10%" align="left"><a accesskey="u" href="sql-commands.html" title="SQL Commands">Up</a></td><th width="60%" align="center">SQL Commands</th><td width="10%" align="right"><a accesskey="h" href="index.html" title="PostgreSQL 16.3 Documentation">Home</a></td><td width="10%" align="right"> <a accesskey="n" href="sql-prepare-transaction.html" title="PREPARE TRANSACTION">Next</a></td></tr></table><hr /></div><div class="refentry" id="SQL-PREPARE"><div class="titlepage"></div><a id="id-1.9.3.159.1" class="indexterm"></a><a id="id-1.9.3.159.2" class="indexterm"></a><div class="refnamediv"><h2><span class="refentrytitle">PREPARE</span></h2><p>PREPARE — prepare a statement for execution</p></div><div class="refsynopsisdiv"><h2>Synopsis</h2><pre class="synopsis">3PREPARE <em class="replaceable"><code>name</code></em> [ ( <em class="replaceable"><code>data_type</code></em> [, ...] ) ] AS <em class="replaceable"><code>statement</code></em>4</pre></div><div class="refsect1" id="id-1.9.3.159.6"><h2>Description</h2><p>5   <code class="command">PREPARE</code> creates a prepared statement. A prepared6   statement is a server-side object that can be used to optimize7   performance. When the <code class="command">PREPARE</code> statement is8   executed, the specified statement is parsed, analyzed, and rewritten.9   When an <code class="command">EXECUTE</code> command is subsequently10   issued, the prepared statement is planned and executed.  This division11   of labor avoids repetitive parse analysis work, while allowing12   the execution plan to depend on the specific parameter values supplied.13  </p><p>14   Prepared statements can take parameters: values that are15   substituted into the statement when it is executed. When creating16   the prepared statement, refer to parameters by position, using17   <code class="literal">$1</code>, <code class="literal">$2</code>, etc. A corresponding list of18   parameter data types can optionally be specified. When a19   parameter's data type is not specified or is declared as20   <code class="literal">unknown</code>, the type is inferred from the context21   in which the parameter is first referenced (if possible). When executing the22   statement, specify the actual values for these parameters in the23   <code class="command">EXECUTE</code> statement.  Refer to <a class="xref" href="sql-execute.html" title="EXECUTE"><span class="refentrytitle">EXECUTE</span></a> for more24   information about that.25  </p><p>26   Prepared statements only last for the duration of the current27   database session. When the session ends, the prepared statement is28   forgotten, so it must be recreated before being used again. This29   also means that a single  prepared statement cannot be used by30   multiple simultaneous database clients; however, each client can create31   their own prepared statement to use.  Prepared statements can be32   manually cleaned up using the <a class="link" href="sql-deallocate.html" title="DEALLOCATE"><code class="command">DEALLOCATE</code></a> command.33  </p><p>34   Prepared statements potentially have the largest performance advantage35   when a single session is being used to execute a large number of similar36   statements. The performance difference will be particularly37   significant if the statements are complex to plan or rewrite, e.g.,38   if the query involves a join of many tables or requires39   the application of several rules. If the statement is relatively simple40   to plan and rewrite but relatively expensive to execute, the41   performance advantage of prepared statements will be less noticeable.42  </p></div><div class="refsect1" id="id-1.9.3.159.7"><h2>Parameters</h2><div class="variablelist"><dl class="variablelist"><dt><span class="term"><em class="replaceable"><code>name</code></em></span></dt><dd><p>43      An arbitrary name given to this particular prepared44      statement. It must be unique within a single session and is45      subsequently used to execute or deallocate a previously prepared46      statement.47     </p></dd><dt><span class="term"><em class="replaceable"><code>data_type</code></em></span></dt><dd><p>48      The data type of a parameter to the prepared statement.  If the49      data type of a particular parameter is unspecified or is50      specified as <code class="literal">unknown</code>, it will be inferred51      from the context in which the parameter is first referenced. To refer to the52      parameters in the prepared statement itself, use53      <code class="literal">$1</code>, <code class="literal">$2</code>, etc.54     </p></dd><dt><span class="term"><em class="replaceable"><code>statement</code></em></span></dt><dd><p>55      Any <code class="command">SELECT</code>, <code class="command">INSERT</code>, <code class="command">UPDATE</code>,56      <code class="command">DELETE</code>, <code class="command">MERGE</code>, or <code class="command">VALUES</code>57      statement.58     </p></dd></dl></div></div><div class="refsect1" id="SQL-PREPARE-NOTES"><h2>Notes</h2><p>59   A prepared statement can be executed with either a <em class="firstterm">generic60   plan</em> or a <em class="firstterm">custom plan</em>.  A generic61   plan is the same across all executions, while a custom plan is generated62   for a specific execution using the parameter values given in that call.63   Use of a generic plan avoids planning overhead, but in some situations64   a custom plan will be much more efficient to execute because the planner65   can make use of knowledge of the parameter values.  (Of course, if the66   prepared statement has no parameters, then this is moot and a generic67   plan is always used.)68  </p><p>69   By default (that is, when <a class="xref" href="runtime-config-query.html#GUC-PLAN-CACHE-MODE">plan_cache_mode</a> is set70   to <code class="literal">auto</code>), the server will automatically choose71   whether to use a generic or custom plan for a prepared statement that72   has parameters.  The current rule for this is that the first five73   executions are done with custom plans and the average estimated cost of74   those plans is calculated.  Then a generic plan is created and its75   estimated cost is compared to the average custom-plan cost.  Subsequent76   executions use the generic plan if its cost is not so much higher than77   the average custom-plan cost as to make repeated replanning seem78   preferable.79  </p><p>80   This heuristic can be overridden, forcing the server to use either81   generic or custom plans, by setting <code class="varname">plan_cache_mode</code>82   to <code class="literal">force_generic_plan</code>83   or <code class="literal">force_custom_plan</code> respectively.84   This setting is primarily useful if the generic plan's cost estimate85   is badly off for some reason, allowing it to be chosen even though86   its actual cost is much more than that of a custom plan.87  </p><p>88   To examine the query plan <span class="productname">PostgreSQL</span> is using89   for a prepared statement, use <a class="link" href="sql-explain.html" title="EXPLAIN"><code class="command">EXPLAIN</code></a>, for example90</p><pre class="programlisting">91EXPLAIN EXECUTE <em class="replaceable"><code>name</code></em>(<em class="replaceable"><code>parameter_values</code></em>);92</pre><p>93   If a generic plan is in use, it will contain parameter symbols94   <code class="literal">$<em class="replaceable"><code>n</code></em></code>, while a custom plan95   will have the supplied parameter values substituted into it.96  </p><p>97   For more information on query planning and the statistics collected98   by <span class="productname">PostgreSQL</span> for that purpose, see99   the <a class="xref" href="sql-analyze.html" title="ANALYZE"><span class="refentrytitle">ANALYZE</span></a>100   documentation.101  </p><p>102   Although the main point of a prepared statement is to avoid repeated parse103   analysis and planning of the statement, <span class="productname">PostgreSQL</span> will104   force re-analysis and re-planning of the statement before using it105   whenever database objects used in the statement have undergone106   definitional (DDL) changes or their planner statistics have107   been updated since the previous use of the prepared108   statement.  Also, if the value of <a class="xref" href="runtime-config-client.html#GUC-SEARCH-PATH">search_path</a> changes109   from one use to the next, the statement will be re-parsed using the new110   <code class="varname">search_path</code>.  (This latter behavior is new as of111   <span class="productname">PostgreSQL</span> 9.3.)  These rules make use of a112   prepared statement semantically almost equivalent to re-submitting the113   same query text over and over, but with a performance benefit if no object114   definitions are changed, especially if the best plan remains the same115   across uses.  An example of a case where the semantic equivalence is not116   perfect is that if the statement refers to a table by an unqualified name,117   and then a new table of the same name is created in a schema appearing118   earlier in the <code class="varname">search_path</code>, no automatic re-parse will occur119   since no object used in the statement changed.  However, if some other120   change forces a re-parse, the new table will be referenced in subsequent121   uses.122  </p><p>123   You can see all prepared statements available in the session by querying the124   <a class="link" href="view-pg-prepared-statements.html" title="54.15. pg_prepared_statements"><code class="structname">pg_prepared_statements</code></a>125   system view.126  </p></div><div class="refsect1" id="SQL-PREPARE-EXAMPLES"><h2>Examples</h2><p>127   Create a prepared statement for an <code class="command">INSERT</code>128   statement, and then execute it:129</p><pre class="programlisting">130PREPARE fooplan (int, text, bool, numeric) AS131    INSERT INTO foo VALUES($1, $2, $3, $4);132EXECUTE fooplan(1, 'Hunter Valley', 't', 200.00);133</pre><p>134  </p><p>135   Create a prepared statement for a <code class="command">SELECT</code>136   statement, and then execute it:137</p><pre class="programlisting">138PREPARE usrrptplan (int) AS139    SELECT * FROM users u, logs l WHERE u.usrid=$1 AND u.usrid=l.usrid140    AND l.date = $2;141EXECUTE usrrptplan(1, current_date);142</pre><p>143 144   In this example, the data type of the second parameter is not specified,145   so it is inferred from the context in which <code class="literal">$2</code> is used.146  </p></div><div class="refsect1" id="id-1.9.3.159.10"><h2>Compatibility</h2><p>147   The SQL standard includes a <code class="command">PREPARE</code> statement,148   but it is only for use in embedded SQL. This version of the149   <code class="command">PREPARE</code> statement also uses a somewhat different150   syntax.151  </p></div><div class="refsect1" id="id-1.9.3.159.11"><h2>See Also</h2><span class="simplelist"><a class="xref" href="sql-deallocate.html" title="DEALLOCATE"><span class="refentrytitle">DEALLOCATE</span></a>, <a class="xref" href="sql-execute.html" title="EXECUTE"><span class="refentrytitle">EXECUTE</span></a></span></div></div><div class="navfooter"><hr /><table width="100%" summary="Navigation footer"><tr><td width="40%" align="left"><a accesskey="p" href="sql-notify.html" title="NOTIFY">Prev</a> </td><td width="20%" align="center"><a accesskey="u" href="sql-commands.html" title="SQL Commands">Up</a></td><td width="40%" align="right"> <a accesskey="n" href="sql-prepare-transaction.html" title="PREPARE TRANSACTION">Next</a></td></tr><tr><td width="40%" align="left" valign="top">NOTIFY </td><td width="20%" align="center"><a accesskey="h" href="index.html" title="PostgreSQL 16.3 Documentation">Home</a></td><td width="40%" align="right" valign="top"> PREPARE TRANSACTION</td></tr></table></div></body></html>