<?xml version="1.0" encoding="UTF-8" standalone="no"?> <!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><div xmlns="http://www.w3.org/TR/xhtml1/transitional" 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 11.4 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></hr></div><div class="refentry" id="SQL-PREPARE"><div class="titlepage"></div><a id="id-1.9.3.158.1" class="indexterm"></a><a id="id-1.9.3.158.2" 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"> PREPARE <em class="replaceable"><code>name</code></em> [ ( <em class="replaceable"><code>data_type</code></em> [, ...] ) ] AS <em class="replaceable"><code>statement</code></em> </pre></div><div class="refsect1" id="id-1.9.3.158.6"><h2>Description</h2><p> <code class="command">PREPARE</code> creates a prepared statement. A prepared statement is a server-side object that can be used to optimize performance. When the <code class="command">PREPARE</code> statement is executed, the specified statement is parsed, analyzed, and rewritten. When an <code class="command">EXECUTE</code> 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 <code class="literal">$1</code>, <code class="literal">$2</code>, 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 <code class="literal">unknown</code>, the type is inferred from the context in which the parameter is first referenced (if possible). When executing the statement, specify the actual values for these parameters in the <code class="command">EXECUTE</code> statement. Refer to <a class="xref" href="sql-execute.html" title="EXECUTE"><span class="refentrytitle">EXECUTE</span></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 class="xref" href="sql-deallocate.html" title="DEALLOCATE"><span class="refentrytitle">DEALLOCATE</span></a> command. </p><p> 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. </p></div><div class="refsect1" id="id-1.9.3.158.7"><h2>Parameters</h2><div class="variablelist"><dl class="variablelist"><dt><span class="term"><em class="replaceable"><code>name</code></em></span></dt><dd><p> 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><span class="term"><em class="replaceable"><code>data_type</code></em></span></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 <code class="literal">unknown</code>, it will be inferred from the context in which the parameter is first referenced. To refer to the parameters in the prepared statement itself, use <code class="literal">$1</code>, <code class="literal">$2</code>, etc. </p></dd><dt><span class="term"><em class="replaceable"><code>statement</code></em></span></dt><dd><p> Any <code class="command">SELECT</code>, <code class="command">INSERT</code>, <code class="command">UPDATE</code>, <code class="command">DELETE</code>, or <code class="command">VALUES</code> statement. </p></dd></dl></div></div><div class="refsect1" id="SQL-PREPARE-NOTES"><h2>Notes</h2><p> Prepared statements can use generic plans rather than re-planning with each set of supplied <code class="command">EXECUTE</code> 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 <code class="command">EXECUTE</code> 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. </p><p> A generic plan assumes that each value supplied to <code class="command">EXECUTE</code> 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. </p><p> To examine the query plan <span class="productname">PostgreSQL</span> is using for a prepared statement, use <a class="xref" href="sql-explain.html" title="EXPLAIN"><span class="refentrytitle">EXPLAIN</span></a>, e.g. <code class="command">EXPLAIN EXECUTE</code>. If a generic plan is in use, it will contain parameter symbols <code class="literal">$<em class="replaceable"><code>n</code></em></code>, 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. </p><p> For more information on query planning and the statistics collected by <span class="productname">PostgreSQL</span> for that purpose, see the <a class="xref" href="sql-analyze.html" title="ANALYZE"><span class="refentrytitle">ANALYZE</span></a> documentation. </p><p> Although the main point of a prepared statement is to avoid repeated parse analysis and planning of the statement, <span class="productname">PostgreSQL</span> 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 <a class="xref" href="runtime-config-client.html#GUC-SEARCH-PATH">search_path</a> changes from one use to the next, the statement will be re-parsed using the new <code class="varname">search_path</code>. (This latter behavior is new as of <span class="productname">PostgreSQL</span> 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 <code class="varname">search_path</code>, 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. </p><p> You can see all prepared statements available in the session by querying the <a class="link" href="view-pg-prepared-statements.html" title="52.76. pg_prepared_statements"><code class="structname">pg_prepared_statements</code></a> system view. </p></div><div class="refsect1" id="SQL-PREPARE-EXAMPLES"><h2>Examples</h2><p> Create a prepared statement for an <code class="command">INSERT</code> 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 <code class="command">SELECT</code> 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 <code class="literal">$2</code> is used. </p></div><div class="refsect1" id="id-1.9.3.158.10"><h2>Compatibility</h2><p> The SQL standard includes a <code class="command">PREPARE</code> statement, but it is only for use in embedded SQL. This version of the <code class="command">PREPARE</code> statement also uses a somewhat different syntax. </p></div><div class="refsect1" id="id-1.9.3.158.11"><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">Prev</a> </td><td width="20%" align="center"><a accesskey="u" href="sql-commands.html">Up</a></td><td width="40%" align="right"> <a accesskey="n" href="sql-prepare-transaction.html">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">Home</a></td><td width="40%" align="right" valign="top"> PREPARE TRANSACTION</td></tr></table></div></body></html>