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Support EXECUTE USING in plpgsql.

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Pavel Stehule, with some improvements by myself.
This commit is contained in:
Tom Lane
2008-04-01 03:51:09 +00:00
parent d5466e38f0
commit e2a8804330
8 changed files with 350 additions and 60 deletions
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64
doc/src/sgml/plpgsql.sgml
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@ -1,4 +1,4 @@
<!-- $PostgreSQL: pgsql/doc/src/sgml/plpgsql.sgml,v 1.125 2008/03/28 00:21:55 tgl Exp $ -->
<!-- $PostgreSQL: pgsql/doc/src/sgml/plpgsql.sgml,v 1.126 2008/04/01 03:51:09 tgl Exp $ -->
<chapter id="plpgsql">
<title><application>PL/pgSQL</application> - <acronym>SQL</acronym> Procedural Language</title>
@ -1005,20 +1005,23 @@ END;
<command>EXECUTE</command> statement is provided:
<synopsis>
EXECUTE <replaceable class="command">command-string</replaceable> <optional> INTO <optional>STRICT</optional> <replaceable>target</replaceable> </optional>;
EXECUTE <replaceable class="command">command-string</replaceable> <optional> INTO <optional>STRICT</optional> <replaceable>target</replaceable> </optional> <optional> USING <replaceable>expression</replaceable> <optional>, ...</optional> </optional>;
</synopsis>
where <replaceable>command-string</replaceable> is an expression
yielding a string (of type <type>text</type>) containing the
command to be executed and <replaceable>target</replaceable> is a
record variable, row variable, or a comma-separated list of
simple variables and record/row fields.
command to be executed. The optional <replaceable>target</replaceable>
is a record variable, a row variable, or a comma-separated list of
simple variables and record/row fields, into which the results of
the command will be stored. The optional <literal>USING</> expressions
supply values to be inserted into the command.
</para>
<para>
No substitution of <application>PL/pgSQL</> variables is done on the
computed command string. Any required variable values must be inserted
in the command string as it is constructed.
in the command string as it is constructed; or you can use parameters
as described below.
</para>
<para>
@ -1046,6 +1049,51 @@ EXECUTE <replaceable class="command">command-string</replaceable> <optional> INT
If the <literal>STRICT</> option is given, an error is reported
unless the query produces exactly one row.
</para>
<para>
The command string can use parameter values, which are referenced
in the command as <literal>$1</>, <literal>$2</>, etc.
These symbols refer to values supplied in the <literal>USING</>
clause. This method is often preferable to inserting data values
into the command string as text: it avoids run-time overhead of
converting the values to text and back, and it is much less prone
to SQL-injection attacks since there is no need for quoting or escaping.
An example is:
<programlisting>
EXECUTE 'SELECT count(*) FROM mytable WHERE inserted_by = $1 AND inserted <= $2'
INTO c
USING checked_user, checked_date;
</programlisting>
Note that parameter symbols can only be used for data values
&mdash; if you want to use dynamically determined table or column
names, you must insert them into the command string textually.
For example, if the preceding query needed to be done against a
dynamically selected table, you could do this:
<programlisting>
EXECUTE 'SELECT count(*) FROM '
|| tabname::regclass
|| ' WHERE inserted_by = $1 AND inserted <= $2'
INTO c
USING checked_user, checked_date;
</programlisting>
</para>
<para>
An <command>EXECUTE</> with a simple constant command string and some
<literal>USING</> parameters, as in the first example above, is
functionally equivalent to just writing the command directly in
<application>PL/pgSQL</application> and allowing replacement of
<application>PL/pgSQL</application> variables to happen automatically.
The important difference is that <command>EXECUTE</> will re-plan
the command on each execution, generating a plan that is specific
to the current parameter values; whereas
<application>PL/pgSQL</application> normally creates a generic plan
and caches it for re-use. In situations where the best plan depends
strongly on the parameter values, <command>EXECUTE</> can be
significantly faster; while when the plan is not sensitive to parameter
values, re-planning will be a waste.
</para>
<para>
<command>SELECT INTO</command> is not currently supported within
@ -1997,7 +2045,7 @@ $$ LANGUAGE plpgsql;
rows:
<synopsis>
<optional> &lt;&lt;<replaceable>label</replaceable>&gt;&gt; </optional>
FOR <replaceable>target</replaceable> IN EXECUTE <replaceable>text_expression</replaceable> LOOP
FOR <replaceable>target</replaceable> IN EXECUTE <replaceable>text_expression</replaceable> <optional> USING <replaceable>expression</replaceable> <optional>, ...</optional> </optional> LOOP
<replaceable>statements</replaceable>
END LOOP <optional> <replaceable>label</replaceable> </optional>;
</synopsis>
@ -2006,6 +2054,8 @@ END LOOP <optional> <replaceable>label</replaceable> </optional>;
on each entry to the <literal>FOR</> loop. This allows the programmer to
choose the speed of a preplanned query or the flexibility of a dynamic
query, just as with a plain <command>EXECUTE</command> statement.
As with <command>EXECUTE</command>, parameter values can be inserted
into the dynamic command via <literal>USING</>.
</para>
</sect2>