1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
|
<?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>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 id="docContent" class="container-fluid col-10"><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 13.4 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">
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>)
</pre></div><div class="refsect1" id="id-1.8.12.8.7.5"><h2>Description</h2><p>
<code class="function">SPI_prepare</code> creates and returns a prepared
statement for the specified command, but doesn't execute the command.
The prepared statement can later be executed repeatedly using
<code class="function">SPI_execute_plan</code>.
</p><p>
When the same or a similar command is to be executed repeatedly, it
is generally advantageous to perform parse analysis only once, and
might furthermore be advantageous to re-use an execution plan for the
command.
<code class="function">SPI_prepare</code> converts a command string into a
prepared statement that encapsulates the results of parse analysis.
The prepared statement also provides a place for caching an execution plan
if it is found that generating a custom plan for each execution is not
helpful.
</p><p>
A prepared command can be generalized by writing parameters
(<code class="literal">$1</code>, <code class="literal">$2</code>, etc.) in place of what would be
constants in a normal command. The actual values of the parameters
are then specified when <code class="function">SPI_execute_plan</code> is called.
This allows the prepared command to be used over a wider range of
situations than would be possible without parameters.
</p><p>
The statement returned by <code class="function">SPI_prepare</code> can be used
only in the current invocation of the C function, since
<code class="function">SPI_finish</code> frees memory allocated for such a
statement. But the statement can be saved for longer using the functions
<code class="function">SPI_keepplan</code> or <code class="function">SPI_saveplan</code>.
</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>
command string
</p></dd><dt><span class="term"><code class="literal">int <em class="parameter"><code>nargs</code></em></code></span></dt><dd><p>
number of input parameters (<code class="literal">$1</code>, <code class="literal">$2</code>, etc.)
</p></dd><dt><span class="term"><code class="literal">Oid * <em class="parameter"><code>argtypes</code></em></code></span></dt><dd><p>
pointer to an array containing the <acronym class="acronym">OID</acronym>s of
the data types of the parameters
</p></dd></dl></div></div><div class="refsect1" id="id-1.8.12.8.7.7"><h2>Return Value</h2><p>
<code class="function">SPI_prepare</code> returns a non-null pointer to an
<code class="type">SPIPlan</code>, which is an opaque struct representing a prepared
statement. On error, <code class="symbol">NULL</code> will be returned,
and <code class="varname">SPI_result</code> will be set to one of the same
error codes used by <code class="function">SPI_execute</code>, except that
it is set to <code class="symbol">SPI_ERROR_ARGUMENT</code> if
<em class="parameter"><code>command</code></em> is <code class="symbol">NULL</code>, or if
<em class="parameter"><code>nargs</code></em> is less than 0, or if <em class="parameter"><code>nargs</code></em> is
greater than 0 and <em class="parameter"><code>argtypes</code></em> is <code class="symbol">NULL</code>.
</p></div><div class="refsect1" id="id-1.8.12.8.7.8"><h2>Notes</h2><p>
If no parameters are defined, a generic plan will be created at the
first use of <code class="function">SPI_execute_plan</code>, and used for all
subsequent executions as well. If there are parameters, the first few uses
of <code class="function">SPI_execute_plan</code> will generate custom plans
that are specific to the supplied parameter values. After enough uses
of the same prepared statement, <code class="function">SPI_execute_plan</code> will
build a generic plan, and if that is not too much more expensive than the
custom plans, it will start using the generic plan instead of re-planning
each time. If this default behavior is unsuitable, you can alter it by
passing the <code class="literal">CURSOR_OPT_GENERIC_PLAN</code> or
<code class="literal">CURSOR_OPT_CUSTOM_PLAN</code> flag to
<code class="function">SPI_prepare_cursor</code>, to force use of generic or custom
plans respectively.
</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.) See <a class="xref" href="sql-prepare.html" title="PREPARE"><span class="refentrytitle">PREPARE</span></a> for more information about the behavior of prepared
statements.
</p><p>
This function should only be called from a connected C function.
</p><p>
<code class="type">SPIPlanPtr</code> is declared as a pointer to an opaque struct type in
<code class="filename">spi.h</code>. It is unwise to try to access its contents
directly, as that makes your code much more likely to break in
future revisions of <span class="productname">PostgreSQL</span>.
</p><p>
The name <code class="type">SPIPlanPtr</code> is somewhat historical, since the data
structure no longer necessarily contains an execution plan.
</p></div></div><div xmlns="http://www.w3.org/TR/xhtml1/transitional" class="navfooter"><hr></hr><table width="100%" summary="Navigation footer"><tr><td width="40%" align="left"><a accesskey="p" href="spi-spi-execute-with-args.html" title="SPI_execute_with_args">Prev</a> </td><td width="20%" align="center"><a accesskey="u" href="spi-interface.html" title="46.1. Interface Functions">Up</a></td><td width="40%" align="right"> <a accesskey="n" href="spi-spi-prepare-cursor.html" title="SPI_prepare_cursor">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" title="PostgreSQL 13.4 Documentation">Home</a></td><td width="40%" align="right" valign="top"> SPI_prepare_cursor</td></tr></table></div></body></html>
|
