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705 lines
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Plaintext
705 lines
25 KiB
Plaintext
<!--
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$Header: /cvsroot/pgsql/doc/src/sgml/pltcl.sgml,v 2.22 2002/09/21 18:32:53 petere Exp $
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-->
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<chapter id="pltcl">
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<title>PL/Tcl - Tcl Procedural Language</title>
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<indexterm zone="pltcl">
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<primary>PL/Tcl</primary>
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</indexterm>
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<indexterm zone="pltcl">
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<primary>Tcl</primary>
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</indexterm>
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<para>
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PL/Tcl is a loadable procedural language for the
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<productname>PostgreSQL</productname> database system
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that enables the Tcl language to be used to write functions and
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trigger procedures.
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</para>
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<para>
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This package was originally written by Jan Wieck.
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</para>
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<!-- **** PL/Tcl overview **** -->
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<sect1 id="pltcl-overview">
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<title>Overview</title>
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<para>
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PL/Tcl offers most of the capabilities a function
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writer has in the C language, except for some restrictions.
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</para>
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<para>
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The good restriction is that everything is executed in a safe
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Tcl interpreter. In addition to the limited command set of safe Tcl, only
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a few commands are available to access the database via SPI and to raise
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messages via <function>elog()</>. There is no way to access internals of the
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database backend or to gain OS-level access under the permissions of the
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<productname>PostgreSQL</productname> user ID, as a C function can do.
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Thus, any unprivileged database user may be
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permitted to use this language.
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</para>
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<para>
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The other, implementation restriction is that Tcl procedures cannot
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be used to create input/output functions for new data types.
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</para>
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<para>
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Sometimes it is desirable to write Tcl functions that are not restricted
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to safe Tcl --- for example, one might want a Tcl function that sends
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mail. To handle these cases, there is a variant of <application>PL/Tcl</> called <literal>PL/TclU</>
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(for untrusted Tcl). This is the exact same language except that a full
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Tcl interpreter is used. <emphasis>If <application>PL/TclU</> is used, it must be
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installed as an untrusted procedural language</emphasis> so that only
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database superusers can create functions in it. The writer of a <application>PL/TclU</>
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function must take care that the function cannot be used to do anything
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unwanted, since it will be able to do anything that could be done by
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a user logged in as the database administrator.
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</para>
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<para>
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The shared object for the <application>PL/Tcl</> and <application>PL/TclU</> call handlers is
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automatically built and installed in the
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<productname>PostgreSQL</productname>
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library directory if Tcl/Tk support is specified
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in the configuration step of the installation procedure. To install
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<application>PL/Tcl</> and/or <application>PL/TclU</> in a particular database, use the
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<filename>createlang</filename> script, for example
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<literal>createlang pltcl <replaceable>dbname</></literal> or
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<literal>createlang pltclu <replaceable>dbname</></literal>.
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</para>
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</sect1>
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<!-- **** PL/Tcl description **** -->
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<sect1 id="pltcl-description">
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<title>Description</title>
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<sect2>
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<title>PL/Tcl Functions and Arguments</title>
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<para>
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To create a function in the <application>PL/Tcl</> language, use the standard syntax
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<programlisting>
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CREATE FUNCTION <replaceable>funcname</replaceable> (<replaceable>argument-types</replaceable>) RETURNS <replaceable>return-type</replaceable> AS '
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# PL/Tcl function body
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' LANGUAGE 'pltcl';
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</programlisting>
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<application>PL/TclU</> is the same, except that the language should be specified as
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<literal>pltclu</>.
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</para>
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<para>
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The body of the function is simply a piece of Tcl script.
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When the function is called, the argument values are passed as
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variables <literal>$1</literal> ... <literal>$n</literal> to the
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Tcl script. The result is returned
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from the Tcl code in the usual way, with a <literal>return</literal>
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statement. For example, a function
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returning the greater of two integer values could be defined as:
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<programlisting>
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CREATE FUNCTION tcl_max (integer, integer) RETURNS integer AS '
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if {$1 > $2} {return $1}
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return $2
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' LANGUAGE 'pltcl' WITH (isStrict);
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</programlisting>
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Note the clause <literal>WITH (isStrict)</>, which saves us from
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having to think about NULL input values: if a NULL is passed, the
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function will not be called at all, but will just return a NULL
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result automatically.
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</para>
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<para>
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In a non-strict function,
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if the actual value of an argument is NULL, the corresponding
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<literal>$n</literal> variable will be set to an empty string.
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To detect whether a particular argument is NULL, use the function
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<literal>argisnull</>. For example, suppose that we wanted <function>tcl_max</function>
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with one null and one non-null argument to return the non-null
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argument, rather than NULL:
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<programlisting>
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CREATE FUNCTION tcl_max (integer, integer) RETURNS integer AS '
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if {[argisnull 1]} {
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if {[argisnull 2]} { return_null }
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return $2
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}
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if {[argisnull 2]} { return $1 }
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if {$1 > $2} {return $1}
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return $2
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' LANGUAGE 'pltcl';
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</programlisting>
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</para>
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<para>
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As shown above,
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to return a NULL value from a PL/Tcl function, execute
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<literal>return_null</literal>. This can be done whether the
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function is strict or not.
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</para>
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<para>
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Composite-type arguments are passed to the procedure as Tcl arrays.
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The element names of the array are the attribute names of the composite
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type. If an attribute in the passed row
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has the NULL value, it will not appear in the array! Here is
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an example that defines the overpaid_2 function (as found in the
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older <productname>PostgreSQL</productname> documentation) in PL/Tcl:
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<programlisting>
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CREATE FUNCTION overpaid_2 (EMP) RETURNS bool AS '
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if {200000.0 < $1(salary)} {
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return "t"
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}
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if {$1(age) < 30 && 100000.0 < $1(salary)} {
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return "t"
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}
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return "f"
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' LANGUAGE 'pltcl';
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</programlisting>
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</para>
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<para>
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There is not currently any support for returning a composite-type
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result value.
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</para>
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</sect2>
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<sect2>
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<title>Data Values in PL/Tcl</title>
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<para>
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The argument values supplied to a PL/Tcl function's script are simply
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the input arguments converted to text form (just as if they had been
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displayed by a SELECT statement). Conversely, the <literal>return</>
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command will accept any string that is acceptable input format for
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the function's declared return type. So, the PL/Tcl programmer can
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manipulate data values as if they were just text.
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</para>
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</sect2>
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<sect2>
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<title>Global Data in PL/Tcl</title>
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<para>
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Sometimes it
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is useful to have some global status data that is held between two
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calls to a procedure or is shared between different procedures.
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This is easily done since
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all PL/Tcl procedures executed in one backend share the same
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safe Tcl interpreter. So, any global Tcl variable is accessible to
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all PL/Tcl procedure calls, and will persist for the duration of the
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SQL client connection. (Note that <application>PL/TclU</> functions likewise share
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global data, but they are in a different Tcl interpreter and cannot
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communicate with PL/Tcl functions.)
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</para>
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<para>
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To help protect PL/Tcl procedures from unintentionally interfering
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with each other, a global
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array is made available to each procedure via the <function>upvar</>
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command. The global name of this variable is the procedure's internal
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name and the local name is <literal>GD</>. It is recommended that
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<literal>GD</> be used
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for private status data of a procedure. Use regular Tcl global variables
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only for values that you specifically intend to be shared among multiple
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procedures.
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</para>
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<para>
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An example of using <literal>GD</> appears in the
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<function>spi_execp</function> example below.
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</para>
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</sect2>
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<sect2>
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<title>Database Access from PL/Tcl</title>
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<para>
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The following commands are available to access the database from
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the body of a PL/Tcl procedure:
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</para>
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<variablelist>
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<varlistentry>
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<term><function>spi_exec</function> <literal>?-count <replaceable>n</replaceable>? ?-array <replaceable>name</replaceable>? <replaceable>query</replaceable> ?<replaceable>loop-body</replaceable>?</literal></term>
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<listitem>
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<para>
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Execute an SQL query given as a string. An error in the query
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causes an error to be raised. Otherwise, the command's return value
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is the number of rows processed (selected, inserted, updated, or
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deleted) by the query, or zero if the query is a utility
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statement. In addition, if the query is a SELECT statement, the
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values of the selected columns are placed in Tcl variables as
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described below.
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</para>
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<para>
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The optional <literal>-count</> value tells
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<function>spi_exec</function> the maximum number of rows
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to process in the query. The effect of this is comparable to
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setting up the query as a cursor and then saying <literal>FETCH n</>.
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</para>
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<para>
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If the query is a <literal>SELECT</> statement, the values of the statement's
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result columns are placed into Tcl variables named after the columns.
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If the <literal>-array</> option is given, the column values are
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instead stored into the named associative array, with the SELECT
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column names used as array indexes.
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</para>
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<para>
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If the query is a SELECT statement and no <replaceable>loop-body</>
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script is given, then only the first row of results are stored into
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Tcl variables; remaining rows, if any, are ignored. No store occurs
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if the
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SELECT returns no rows (this case can be detected by checking the
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result of <function>spi_exec</function>). For example,
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<programlisting>
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spi_exec "SELECT count(*) AS cnt FROM pg_proc"
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</programlisting>
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will set the Tcl variable <literal>$cnt</> to the number of rows in
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the <structname>pg_proc</> system catalog.
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</para>
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<para>
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If the optional <replaceable>loop-body</> argument is given, it is
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a piece of Tcl script that is executed once for each row in the
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SELECT result (note: <replaceable>loop-body</> is ignored if the given
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query is not a SELECT). The values of the current row's fields
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are stored into Tcl variables before each iteration. For example,
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<programlisting>
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spi_exec -array C "SELECT * FROM pg_class" {
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elog DEBUG "have table $C(relname)"
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}
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</programlisting>
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will print a DEBUG log message for every row of pg_class. This
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feature works similarly to other Tcl looping constructs; in
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particular <literal>continue</> and <literal>break</> work in the
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usual way inside the loop body.
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</para>
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<para>
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If a field of a SELECT result is NULL, the target
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variable for it is <quote>unset</> rather than being set.
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</para>
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</listitem>
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</varlistentry>
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<varlistentry>
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<term><function>spi_prepare</function> <replaceable>query</replaceable> <replaceable>typelist</replaceable></term>
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<listitem>
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<para>
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Prepares and saves a query plan for later execution. The saved plan
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will be retained for the life of the current backend.
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</para>
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<para>
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The query may use <firstterm>arguments</>, which are placeholders for
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values to be supplied whenever the plan is actually executed.
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In the query string, refer to arguments
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by the symbols <literal>$1</literal> ... <literal>$n</literal>.
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If the query uses arguments, the names of the argument types
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must be given as a Tcl list. (Write an empty list for
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<replaceable>typelist</replaceable> if no arguments are used.)
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Presently, the argument types must be identified by the internal
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type names shown in pg_type; for example <literal>int4</> not
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<literal>integer</>.
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</para>
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<para>
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The return value from <function>spi_prepare</function> is a query ID
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to be used in subsequent calls to <function>spi_execp</function>. See
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<function>spi_execp</function> for an example.
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</para>
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</listitem>
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</varlistentry>
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<varlistentry>
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<term><function>spi_execp</> <literal>?-count <replaceable>n</replaceable>? ?-array <replaceable>name</replaceable>? ?-nulls <replaceable>string</replaceable>? <replaceable>queryid</replaceable> ?<replaceable>value-list</replaceable>? ?<replaceable>loop-body</replaceable>?</literal></term>
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<listitem>
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<para>
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Execute a query previously prepared with <function>spi_prepare</>.
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<replaceable>queryid</replaceable> is the ID returned by
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<function>spi_prepare</>. If the query references arguments,
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a <replaceable>value-list</replaceable> must be supplied: this
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is a Tcl list of actual values for the arguments. This must be
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the same length as the argument type list previously given to
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<function>spi_prepare</>. Omit <replaceable>value-list</replaceable>
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if the query has no arguments.
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</para>
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<para>
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The optional value for <literal>-nulls</> is a string of spaces and
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<literal>'n'</> characters telling <function>spi_execp</function>
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which of the arguments are null values. If given, it must have exactly the
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same length as the <replaceable>value-list</replaceable>. If it
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is not given, all the argument values are non-NULL.
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</para>
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<para>
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Except for the way in which the query and its arguments are specified,
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<function>spi_execp</> works just like <function>spi_exec</>.
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The <literal>-count</>, <literal>-array</>, and
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<replaceable>loop-body</replaceable> options are the same,
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and so is the result value.
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</para>
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<para>
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Here's an example of a PL/Tcl function using a prepared plan:
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<programlisting>
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CREATE FUNCTION t1_count(integer, integer) RETURNS integer AS '
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if {![ info exists GD(plan) ]} {
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# prepare the saved plan on the first call
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set GD(plan) [ spi_prepare \\
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"SELECT count(*) AS cnt FROM t1 WHERE num >= \\$1 AND num <= \\$2" \\
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[ list int4 int4 ] ]
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}
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spi_execp -count 1 $GD(plan) [ list $1 $2 ]
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return $cnt
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' LANGUAGE 'pltcl';
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</programlisting>
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Note that each backslash that Tcl should see must be doubled when
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we type in the function, since the main parser processes
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backslashes too in CREATE FUNCTION. We need backslashes inside
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the query string given to <function>spi_prepare</> to ensure that
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the <literal>$n</> markers will be passed through to
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<function>spi_prepare</> as-is, and not
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replaced by Tcl variable substitution.
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</para>
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</listitem>
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</varlistentry>
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<varlistentry>
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<indexterm>
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<primary>spi_lastoid</primary>
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</indexterm>
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<term><function>spi_lastoid</></term>
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<listitem>
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<para>
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Returns the OID of the row inserted by the last
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<function>spi_exec</>'d or <function>spi_execp</>'d query,
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if that query was a single-row INSERT. (If not, you get zero.)
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</para>
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</listitem>
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</varlistentry>
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<varlistentry>
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<term><function>quote</> <replaceable>string</replaceable></term>
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<listitem>
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<para>
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Duplicates all occurrences of single quote and backslash characters
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in the given string. This may be used to safely quote strings
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that are to be inserted into SQL queries given
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to <function>spi_exec</function> or
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<function>spi_prepare</function>.
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For example, think about a query string like
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<programlisting>
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"SELECT '$val' AS ret"
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</programlisting>
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where the Tcl variable val actually contains
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<literal>doesn't</literal>. This would result
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in the final query string
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<programlisting>
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SELECT 'doesn't' AS ret
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</programlisting>
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which would cause a parse error during
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<function>spi_exec</function> or
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<function>spi_prepare</function>.
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The submitted query should contain
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<programlisting>
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SELECT 'doesn''t' AS ret
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</programlisting>
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which can be formed in PL/Tcl as
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<programlisting>
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"SELECT '[ quote $val ]' AS ret"
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</programlisting>
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One advantage of <function>spi_execp</function> is that you don't
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have to quote argument values like this, since the arguments are never
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parsed as part of an SQL query string.
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</para>
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</listitem>
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</varlistentry>
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<varlistentry>
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<indexterm>
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<primary>elog</primary>
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</indexterm>
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<term><function>elog</> <replaceable>level</replaceable> <replaceable>msg</replaceable></term>
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<listitem>
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<para>
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Emit a log or error message. Possible levels are
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<literal>DEBUG</>, <literal>LOG</>, <literal>INFO</>,
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<literal>NOTICE</>, <literal>WARNING</>, <literal>ERROR</>, and
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<literal>FATAL</>. Most simply emit the given message just like
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the <literal>elog</> backend C function. <literal>ERROR</>
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raises an error condition: further execution of the function is
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abandoned, and the current transaction is aborted.
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<literal>FATAL</> aborts the transaction and causes the current
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backend to shut down (there is probably no good reason to use
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this error level in PL/Tcl functions, but it's provided for
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completeness).
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</para>
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</listitem>
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</varlistentry>
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</variablelist>
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</sect2>
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<sect2>
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<title>Trigger Procedures in PL/Tcl</title>
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<indexterm>
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<primary>triggers</primary>
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<secondary>in PL/Tcl</secondary>
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</indexterm>
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<para>
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Trigger procedures can be written in PL/Tcl. As is customary in
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<productname>PostgreSQL</productname>, a procedure that's to be called
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as a trigger must be declared as a function with no arguments
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and a return type of <literal>trigger</>.
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</para>
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<para>
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The information from the trigger manager is passed to the procedure body
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in the following variables:
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<variablelist>
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<varlistentry>
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<term><replaceable class="Parameter">$TG_name</replaceable></term>
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<listitem>
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<para>
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The name of the trigger from the CREATE TRIGGER statement.
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</para>
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</listitem>
|
|
</varlistentry>
|
|
|
|
<varlistentry>
|
|
<term><replaceable class="Parameter">$TG_relid</replaceable></term>
|
|
<listitem>
|
|
<para>
|
|
The object ID of the table that caused the trigger procedure
|
|
to be invoked.
|
|
</para>
|
|
</listitem>
|
|
</varlistentry>
|
|
|
|
<varlistentry>
|
|
<term><replaceable class="Parameter">$TG_relatts</replaceable></term>
|
|
<listitem>
|
|
<para>
|
|
A Tcl list of the table field names, prefixed with an empty list
|
|
element. So looking up an element name in the list with <application>Tcl</>'s
|
|
<function>lsearch</> command returns the element's number starting
|
|
with 1 for the first column, the same way the fields are customarily
|
|
numbered in <productname>PostgreSQL</productname>.
|
|
</para>
|
|
</listitem>
|
|
</varlistentry>
|
|
|
|
<varlistentry>
|
|
<term><replaceable class="Parameter">$TG_when</replaceable></term>
|
|
<listitem>
|
|
<para>
|
|
The string <literal>BEFORE</> or <literal>AFTER</> depending on the
|
|
type of trigger call.
|
|
</para>
|
|
</listitem>
|
|
</varlistentry>
|
|
|
|
<varlistentry>
|
|
<term><replaceable class="Parameter">$TG_level</replaceable></term>
|
|
<listitem>
|
|
<para>
|
|
The string <literal>ROW</> or <literal>STATEMENT</> depending on the
|
|
type of trigger call.
|
|
</para>
|
|
</listitem>
|
|
</varlistentry>
|
|
|
|
<varlistentry>
|
|
<term><replaceable class="Parameter">$TG_op</replaceable></term>
|
|
<listitem>
|
|
<para>
|
|
The string <literal>INSERT</>, <literal>UPDATE</> or
|
|
<literal>DELETE</> depending on the type of trigger call.
|
|
</para>
|
|
</listitem>
|
|
</varlistentry>
|
|
|
|
<varlistentry>
|
|
<term><replaceable class="Parameter">$NEW</replaceable></term>
|
|
<listitem>
|
|
<para>
|
|
An associative array containing the values of the new table row for
|
|
INSERT/UPDATE actions, or empty for DELETE. The array is indexed
|
|
by field name. Fields that are NULL will not appear in the array!
|
|
</para>
|
|
</listitem>
|
|
</varlistentry>
|
|
|
|
<varlistentry>
|
|
<term><replaceable class="Parameter">$OLD</replaceable></term>
|
|
<listitem>
|
|
<para>
|
|
An associative array containing the values of the old table row for
|
|
UPDATE/DELETE actions, or empty for INSERT. The array is indexed
|
|
by field name. Fields that are NULL will not appear in the array!
|
|
</para>
|
|
</listitem>
|
|
</varlistentry>
|
|
|
|
<varlistentry>
|
|
<term><replaceable class="Parameter">$args</replaceable></term>
|
|
<listitem>
|
|
<para>
|
|
A Tcl list of the arguments to the procedure as given in the
|
|
CREATE TRIGGER statement. These arguments are also accessible as
|
|
<literal>$1</literal> ... <literal>$n</literal> in the procedure body.
|
|
</para>
|
|
</listitem>
|
|
</varlistentry>
|
|
|
|
</variablelist>
|
|
</para>
|
|
|
|
<para>
|
|
The return value from a trigger procedure can be one of the strings
|
|
<literal>OK</> or <literal>SKIP</>, or a list as returned by the
|
|
<literal>array get</> Tcl command. If the return value is <literal>OK</>,
|
|
the operation (INSERT/UPDATE/DELETE) that fired the trigger will proceed
|
|
normally. <literal>SKIP</> tells the trigger manager to silently suppress
|
|
the operation for this row. If a list is returned, it tells PL/Tcl to
|
|
return a modified row to the trigger manager that will be inserted
|
|
instead of the one given in $NEW (this works for INSERT/UPDATE
|
|
only). Needless to say that all this is only meaningful when the trigger
|
|
is BEFORE and FOR EACH ROW; otherwise the return value is ignored.
|
|
</para>
|
|
<para>
|
|
Here's a little example trigger procedure that forces an integer value
|
|
in a table to keep track of the number of updates that are performed on the
|
|
row. For new rows inserted, the value is initialized to 0 and then
|
|
incremented on every update operation:
|
|
|
|
<programlisting>
|
|
CREATE FUNCTION trigfunc_modcount() RETURNS TRIGGER AS '
|
|
switch $TG_op {
|
|
INSERT {
|
|
set NEW($1) 0
|
|
}
|
|
UPDATE {
|
|
set NEW($1) $OLD($1)
|
|
incr NEW($1)
|
|
}
|
|
default {
|
|
return OK
|
|
}
|
|
}
|
|
return [array get NEW]
|
|
' LANGUAGE 'pltcl';
|
|
|
|
CREATE TABLE mytab (num integer, description text, modcnt integer);
|
|
|
|
CREATE TRIGGER trig_mytab_modcount BEFORE INSERT OR UPDATE ON mytab
|
|
FOR EACH ROW EXECUTE PROCEDURE trigfunc_modcount('modcnt');
|
|
</programlisting>
|
|
|
|
Notice that the trigger procedure itself does not know the column
|
|
name; that's supplied from the trigger arguments. This lets the
|
|
trigger procedure be re-used with different tables.
|
|
</para>
|
|
</sect2>
|
|
|
|
<sect2>
|
|
<title> Modules and the <function>unknown</> command</title>
|
|
<para>
|
|
PL/Tcl has support for auto-loading Tcl code when used.
|
|
It recognizes a special table, <literal>pltcl_modules</>, which
|
|
is presumed to contain modules of Tcl code. If this table
|
|
exists, the module <literal>unknown</> is fetched from the table
|
|
and loaded into the Tcl interpreter immediately after creating
|
|
the interpreter.
|
|
</para>
|
|
<para>
|
|
While the <literal>unknown</> module could actually contain any
|
|
initialization script you need, it normally defines a Tcl
|
|
<quote>unknown</> procedure that is invoked whenever Tcl does
|
|
not recognize an invoked procedure name. <application>PL/Tcl</>'s standard version
|
|
of this procedure tries to find a module in <literal>pltcl_modules</>
|
|
that will define the required procedure. If one is found, it is
|
|
loaded into the interpreter, and then execution is allowed to
|
|
proceed with the originally attempted procedure call. A
|
|
secondary table <literal>pltcl_modfuncs</> provides an index of
|
|
which functions are defined by which modules, so that the lookup
|
|
is reasonably quick.
|
|
</para>
|
|
<para>
|
|
The <productname>PostgreSQL</productname> distribution includes
|
|
support scripts to maintain these tables:
|
|
<command>pltcl_loadmod</>, <command>pltcl_listmod</>,
|
|
<command>pltcl_delmod</>, as well as source for the standard
|
|
unknown module <filename>share/unknown.pltcl</>. This module
|
|
must be loaded
|
|
into each database initially to support the autoloading mechanism.
|
|
</para>
|
|
<para>
|
|
The tables <literal>pltcl_modules</> and <literal>pltcl_modfuncs</>
|
|
must be readable by all, but it is wise to make them owned and
|
|
writable only by the database administrator.
|
|
</para>
|
|
</sect2>
|
|
|
|
<sect2>
|
|
<title>Tcl Procedure Names</title>
|
|
|
|
<para>
|
|
In <productname>PostgreSQL</productname>, one and the
|
|
same function name can be used for
|
|
different functions as long as the number of arguments or their types
|
|
differ. Tcl, however, requires all procedure names to be distinct.
|
|
PL/Tcl deals with this by making the internal Tcl procedure names contain
|
|
the object
|
|
ID of the procedure's <structname>pg_proc</> row as part of their name. Thus,
|
|
<productname>PostgreSQL</productname> functions with the same name
|
|
and different argument types will be different Tcl procedures too. This
|
|
is not normally a concern for a PL/Tcl programmer, but it might be visible
|
|
when debugging.
|
|
</para>
|
|
|
|
</sect2>
|
|
|
|
</sect1>
|
|
</chapter>
|
|
|
|
<!-- Keep this comment at the end of the file
|
|
Local variables:
|
|
mode:sgml
|
|
sgml-omittag:nil
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sgml-shorttag:t
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sgml-minimize-attributes:nil
|
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|
|
sgml-indent-step:1
|
|
sgml-indent-data:t
|
|
sgml-parent-document:nil
|
|
sgml-default-dtd-file:"./reference.ced"
|
|
sgml-exposed-tags:nil
|
|
sgml-local-catalogs:("/usr/lib/sgml/catalog")
|
|
sgml-local-ecat-files:nil
|
|
End:
|
|
-->
|