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
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
|
<!--
doc/src/sgml/ref/prepare.sgml
PostgreSQL documentation
-->
<refentry id="SQL-PREPARE">
<refmeta>
<refentrytitle>PREPARE</refentrytitle>
<manvolnum>7</manvolnum>
<refmiscinfo>SQL - Language Statements</refmiscinfo>
</refmeta>
<refnamediv>
<refname>PREPARE</refname>
<refpurpose>prepare a statement for execution</refpurpose>
</refnamediv>
<indexterm zone="sql-prepare">
<primary>PREPARE</primary>
</indexterm>
<indexterm zone="sql-prepare">
<primary>prepared statements</primary>
<secondary>creating</secondary>
</indexterm>
<refsynopsisdiv>
<synopsis>
PREPARE <replaceable class="PARAMETER">name</replaceable> [ ( <replaceable class="PARAMETER">data_type</replaceable> [, ...] ) ] AS <replaceable class="PARAMETER">statement</replaceable>
</synopsis>
</refsynopsisdiv>
<refsect1>
<title>Description</title>
<para>
<command>PREPARE</command> creates a prepared statement. A prepared
statement is a server-side object that can be used to optimize
performance. When the <command>PREPARE</command> statement is
executed, the specified statement is parsed, analyzed, and rewritten.
When an <command>EXECUTE</command> 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.
</para>
<para>
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
<literal>$1</>, <literal>$2</>, 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
<literal>unknown</literal>, 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
<command>EXECUTE</command> statement. Refer to <xref
linkend="sql-execute"> for more
information about that.
</para>
<para>
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 <xref linkend="sql-deallocate"> command.
</para>
<para>
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.
</para>
</refsect1>
<refsect1>
<title>Parameters</title>
<variablelist>
<varlistentry>
<term><replaceable class="PARAMETER">name</replaceable></term>
<listitem>
<para>
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.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><replaceable class="PARAMETER">data_type</replaceable></term>
<listitem>
<para>
The data type of a parameter to the prepared statement. If the
data type of a particular parameter is unspecified or is
specified as <literal>unknown</literal>, it will be inferred
from the context in which the parameter is used. To refer to the
parameters in the prepared statement itself, use
<literal>$1</literal>, <literal>$2</literal>, etc.
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><replaceable class="PARAMETER">statement</replaceable></term>
<listitem>
<para>
Any <command>SELECT</>, <command>INSERT</>, <command>UPDATE</>,
<command>DELETE</>, or <command>VALUES</> statement.
</para>
</listitem>
</varlistentry>
</variablelist>
</refsect1>
<refsect1>
<title>Notes</title>
<para>
If a prepared statement is executed enough times, the server may eventually
decide to save and re-use a generic plan rather than re-planning each time.
This will occur immediately if the prepared statement has no parameters;
otherwise it occurs only if the generic plan appears to be not much more
expensive than a plan that depends on specific parameter values.
Typically, a generic plan will be selected only if the query's performance
is estimated to be fairly insensitive to the specific parameter values
supplied.
</para>
<para>
To examine the query plan <productname>PostgreSQL</productname> is using
for a prepared statement, use <xref linkend="sql-explain">.
If a generic plan is in use, it will contain parameter symbols
<literal>$<replaceable>n</></literal>, while a custom plan will have the
current actual parameter values substituted into it.
</para>
<para>
For more information on query planning and the statistics collected
by <productname>PostgreSQL</productname> for that purpose, see
the <xref linkend="sql-analyze">
documentation.
</para>
<para>
You can see all prepared statements available in the session by querying the
<link linkend="view-pg-prepared-statements"><structname>pg_prepared_statements</structname></link>
system view.
</para>
</refsect1>
<refsect1 id="sql-prepare-examples">
<title id="sql-prepare-examples-title">Examples</title>
<para>
Create a prepared statement for an <command>INSERT</command>
statement, and then execute it:
<programlisting>
PREPARE fooplan (int, text, bool, numeric) AS
INSERT INTO foo VALUES($1, $2, $3, $4);
EXECUTE fooplan(1, 'Hunter Valley', 't', 200.00);
</programlisting>
</para>
<para>
Create a prepared statement for a <command>SELECT</command>
statement, and then execute it:
<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);
</programlisting>
Note that the data type of the second parameter is not specified,
so it is inferred from the context in which <literal>$2</> is used.
</para>
</refsect1>
<refsect1>
<title>Compatibility</title>
<para>
The SQL standard includes a <command>PREPARE</command> statement,
but it is only for use in embedded SQL. This version of the
<command>PREPARE</command> statement also uses a somewhat different
syntax.
</para>
</refsect1>
<refsect1>
<title>See Also</title>
<simplelist type="inline">
<member><xref linkend="sql-deallocate"></member>
<member><xref linkend="sql-execute"></member>
</simplelist>
</refsect1>
</refentry>
|
