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Green_GoodERP18_FSJ_Standard/PostgreSQL/doc/postgresql/html/spi-spi-prepare.html

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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>SPI_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 V1.79.1" /><link rel="prev" href="spi-spi-execute-with-args.html" title="SPI_execute_with_args" /><link rel="next" href="spi-spi-prepare-cursor.html" title="SPI_prepare_cursor" /></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">SPI_prepare</th></tr><tr><td width="10%" align="left"><a accesskey="p" href="spi-spi-execute-with-args.html" title="SPI_execute_with_args">Prev</a> </td><td width="10%" align="left"><a accesskey="u" href="spi-interface.html" title="46.1. Interface Functions">Up</a></td><th width="60%" align="center">46.1. Interface Functions</th><td width="10%" align="right"><a accesskey="h" href="index.html" title="PostgreSQL 12.4 Documentation">Home</a></td><td width="10%" align="right"> <a accesskey="n" href="spi-spi-prepare-cursor.html" title="SPI_prepare_cursor">Next</a></td></tr></table><hr></hr></div><div class="refentry" id="SPI-SPI-PREPARE"><div class="titlepage"></div><a id="id-1.8.12.8.7.1" class="indexterm"></a><div class="refnamediv"><h2><span class="refentrytitle">SPI_prepare</span></h2><p>SPI_prepare — prepare a statement, without executing it yet</p></div><div class="refsynopsisdiv"><h2>Synopsis</h2><pre class="synopsis">

  3. SPIPlanPtr SPI_prepare(const char * <em class="parameter"><code>command</code></em>, int <em class="parameter"><code>nargs</code></em>, Oid * <em class="parameter"><code>argtypes</code></em>)

  4. </pre></div><div class="refsect1" id="id-1.8.12.8.7.5"><h2>Description</h2><p>

  5.    <code class="function">SPI_prepare</code> creates and returns a prepared

  6.    statement for the specified command, but doesn't execute the command.

  7.    The prepared statement can later be executed repeatedly using

  8.    <code class="function">SPI_execute_plan</code>.

  9.   </p><p>

  10.    When the same or a similar command is to be executed repeatedly, it

  11.    is generally advantageous to perform parse analysis only once, and

  12.    might furthermore be advantageous to re-use an execution plan for the

  13.    command.

  14.    <code class="function">SPI_prepare</code> converts a command string into a

  15.    prepared statement that encapsulates the results of parse analysis.

  16.    The prepared statement also provides a place for caching an execution plan

  17.    if it is found that generating a custom plan for each execution is not

  18.    helpful.

  19.   </p><p>

  20.    A prepared command can be generalized by writing parameters

  21.    (<code class="literal">$1</code>, <code class="literal">$2</code>, etc.) in place of what would be

  22.    constants in a normal command.  The actual values of the parameters

  23.    are then specified when <code class="function">SPI_execute_plan</code> is called.

  24.    This allows the prepared command to be used over a wider range of

  25.    situations than would be possible without parameters.

  26.   </p><p>

  27.    The statement returned by <code class="function">SPI_prepare</code> can be used

  28.    only in the current invocation of the C function, since

  29.    <code class="function">SPI_finish</code> frees memory allocated for such a

  30.    statement.  But the statement can be saved for longer using the functions

  31.    <code class="function">SPI_keepplan</code> or <code class="function">SPI_saveplan</code>.

  32.   </p></div><div class="refsect1" id="id-1.8.12.8.7.6"><h2>Arguments</h2><div class="variablelist"><dl class="variablelist"><dt><span class="term"><code class="literal">const char * <em class="parameter"><code>command</code></em></code></span></dt><dd><p>

  33.       command string

  34.      </p></dd><dt><span class="term"><code class="literal">int <em class="parameter"><code>nargs</code></em></code></span></dt><dd><p>

  35.       number of input parameters (<code class="literal">$1</code>, <code class="literal">$2</code>, etc.)

  36.      </p></dd><dt><span class="term"><code class="literal">Oid * <em class="parameter"><code>argtypes</code></em></code></span></dt><dd><p>

  37.       pointer to an array containing the <acronym class="acronym">OID</acronym>s of

  38.       the data types of the parameters

  39.      </p></dd></dl></div></div><div class="refsect1" id="id-1.8.12.8.7.7"><h2>Return Value</h2><p>

  40.    <code class="function">SPI_prepare</code> returns a non-null pointer to an

  41.    <code class="type">SPIPlan</code>, which is an opaque struct representing a prepared

  42.    statement.  On error, <code class="symbol">NULL</code> will be returned,

  43.    and <code class="varname">SPI_result</code> will be set to one of the same

  44.    error codes used by <code class="function">SPI_execute</code>, except that

  45.    it is set to <code class="symbol">SPI_ERROR_ARGUMENT</code> if

  46.    <em class="parameter"><code>command</code></em> is <code class="symbol">NULL</code>, or if

  47.    <em class="parameter"><code>nargs</code></em> is less than 0, or if <em class="parameter"><code>nargs</code></em> is

  48.    greater than 0 and <em class="parameter"><code>argtypes</code></em> is <code class="symbol">NULL</code>.

  49.   </p></div><div class="refsect1" id="id-1.8.12.8.7.8"><h2>Notes</h2><p>

  50.    If no parameters are defined, a generic plan will be created at the

  51.    first use of <code class="function">SPI_execute_plan</code>, and used for all

  52.    subsequent executions as well.  If there are parameters, the first few uses

  53.    of <code class="function">SPI_execute_plan</code> will generate custom plans

  54.    that are specific to the supplied parameter values.  After enough uses

  55.    of the same prepared statement, <code class="function">SPI_execute_plan</code> will

  56.    build a generic plan, and if that is not too much more expensive than the

  57.    custom plans, it will start using the generic plan instead of re-planning

  58.    each time.  If this default behavior is unsuitable, you can alter it by

  59.    passing the <code class="literal">CURSOR_OPT_GENERIC_PLAN</code> or

  60.    <code class="literal">CURSOR_OPT_CUSTOM_PLAN</code> flag to

  61.    <code class="function">SPI_prepare_cursor</code>, to force use of generic or custom

  62.    plans respectively.

  63.   </p><p>

  64.    Although the main point of a prepared statement is to avoid repeated parse

  65.    analysis and planning of the statement, <span class="productname">PostgreSQL</span> will

  66.    force re-analysis and re-planning of the statement before using it

  67.    whenever database objects used in the statement have undergone

  68.    definitional (DDL) changes since the previous use of the prepared

  69.    statement.  Also, if the value of <a class="xref" href="runtime-config-client.html#GUC-SEARCH-PATH">search_path</a> changes

  70.    from one use to the next, the statement will be re-parsed using the new

  71.    <code class="varname">search_path</code>.  (This latter behavior is new as of

  72.    <span class="productname">PostgreSQL</span> 9.3.)  See <a class="xref" href="sql-prepare.html" title="PREPARE"><span class="refentrytitle">PREPARE</span></a> for more information about the behavior of prepared

  73.    statements.

  74.   </p><p>

  75.    This function should only be called from a connected C function.

  76.   </p><p>

  77.    <code class="type">SPIPlanPtr</code> is declared as a pointer to an opaque struct type in

  78.    <code class="filename">spi.h</code>.  It is unwise to try to access its contents

  79.    directly, as that makes your code much more likely to break in

  80.    future revisions of <span class="productname">PostgreSQL</span>.

  81.   </p><p>

  82.    The name <code class="type">SPIPlanPtr</code> is somewhat historical, since the data

  83.    structure no longer necessarily contains an execution plan.

  84.   </p></div></div><div class="navfooter"><hr /><table width="100%" summary="Navigation footer"><tr><td width="40%" align="left"><a accesskey="p" href="spi-spi-execute-with-args.html">Prev</a> </td><td width="20%" align="center"><a accesskey="u" href="spi-interface.html">Up</a></td><td width="40%" align="right"> <a accesskey="n" href="spi-spi-prepare-cursor.html">Next</a></td></tr><tr><td width="40%" align="left" valign="top">SPI_execute_with_args </td><td width="20%" align="center"><a accesskey="h" href="index.html">Home</a></td><td width="40%" align="right" valign="top"> SPI_prepare_cursor</td></tr></table></div></body></html>
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