Overhaul PL/Tcl documentation.
This commit is contained in:
Tom Lane
parent
35a5129a08
commit
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@ -1,5 +1,5 @@
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<!--
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$Header: /cvsroot/pgsql/doc/src/sgml/pltcl.sgml,v 2.17 2001/11/21 05:53:41 thomas Exp $
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$Header: /cvsroot/pgsql/doc/src/sgml/pltcl.sgml,v 2.18 2002/01/23 21:08:17 tgl Exp $
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-->
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<chapter id="pltcl">
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@ -16,7 +16,7 @@ $Header: /cvsroot/pgsql/doc/src/sgml/pltcl.sgml,v 2.17 2001/11/21 05:53:41 thoma
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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 create functions and
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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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@ -66,7 +66,9 @@ $Header: /cvsroot/pgsql/doc/src/sgml/pltcl.sgml,v 2.17 2001/11/21 05:53:41 thoma
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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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PL/Tcl and/or PL/TclU in a particular database, use the
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<filename>createlang</filename> script.
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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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@ -76,23 +78,7 @@ $Header: /cvsroot/pgsql/doc/src/sgml/pltcl.sgml,v 2.17 2001/11/21 05:53:41 thoma
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<title>Description</title>
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<sect2>
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<title><productname>PostgreSQL</productname> Functions and Tcl Procedure Names</title>
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<para>
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In <productname>PostgreSQL</productname>, one and the
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same function name can be used for
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different functions as long as the number of arguments or their types
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differ. This would collide with Tcl procedure names. To offer the same
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flexibility in PL/Tcl, the internal Tcl procedure names contain the object
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ID of the procedure's pg_proc row as part of their name. Thus, different
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argtype versions of the same <productname>PostgreSQL</productname>
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function are different for Tcl too.
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</para>
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</sect2>
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<sect2>
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<title>Defining Functions in PL/Tcl</title>
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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 PL/Tcl language, use the standard syntax
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@ -103,32 +89,68 @@ CREATE FUNCTION <replaceable>funcname</replaceable> (<replaceable>argument-types
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' LANGUAGE 'pltcl';
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</programlisting>
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When the function is called, the arguments are given as
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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 procedure body. The result is returned
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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 higher of two int4 values could be defined as:
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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 (int4, int4) RETURNS int4 AS '
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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 tcl_max
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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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To return a NULL value from a PL/Tcl function, execute
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<literal>return_null</literal>.
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</para>
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<para>
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Composite type arguments are given to the procedure as Tcl arrays.
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The element names
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in the array are the attribute names of the composite
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type. If an attribute in the actual row
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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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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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@ -143,27 +165,298 @@ CREATE FUNCTION overpaid_2 (EMP) RETURNS bool AS '
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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 (especially when using the SPI functions described later) it
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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. This is easily done since
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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.
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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 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 unwanted side effects,
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an array is made available to each procedure via the <function>upvar</>
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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 GD. It is recommended that GD be used
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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 SELECT statement, the values of the SELECT'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 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 NULLs. 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 <literal>DEBUG</>,
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<literal>NOTICE</>, <literal>ERROR</>, and <literal>FATAL</>.
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<literal>DEBUG</> and <literal>NOTICE</> simply emit the given message
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into the postmaster log (and send it to the client too, in the case of
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<literal>NOTICE</>). <literal>ERROR</> raises an error condition:
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further execution of the function is abandoned, and the current
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transaction is aborted. <literal>FATAL</> aborts the transaction and
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causes the current backend to shut down (there is probably no good
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reason to use this error level in PL/Tcl functions, but it's provided
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for 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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@ -175,13 +468,13 @@ CREATE FUNCTION overpaid_2 (EMP) RETURNS bool AS '
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</indexterm>
|
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<para>
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Trigger procedures are defined in <productname>PostgreSQL</productname>
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as functions without
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arguments and a return type of opaque. And so are they in the PL/Tcl
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language.
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Trigger procedures can be written in PL/Tcl. As is customary in
|
||||
<productname>PostgreSQL</productname>, a procedure that's to be called
|
||||
as a trigger must be declared as a function with no arguments
|
||||
and a return type of <literal>opaque</>.
|
||||
</para>
|
||||
<para>
|
||||
The information from the trigger manager is given to the procedure body
|
||||
The information from the trigger manager is passed to the procedure body
|
||||
in the following variables:
|
||||
|
||||
<variablelist>
|
||||
@ -209,10 +502,11 @@ CREATE FUNCTION overpaid_2 (EMP) RETURNS bool AS '
|
||||
<term><replaceable class="Parameter">$TG_relatts</replaceable></term>
|
||||
<listitem>
|
||||
<para>
|
||||
A Tcl list of the tables field names prefixed with an empty list element.
|
||||
So looking up an element name in the list with the <function>lsearch</> Tcl command
|
||||
returns the same positive number starting from 1 as the fields are numbered
|
||||
in the pg_attribute system catalog.
|
||||
A Tcl list of the table field names, prefixed with an empty list
|
||||
element. So looking up an element name in the list with 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>
|
||||
@ -221,7 +515,8 @@ CREATE FUNCTION overpaid_2 (EMP) RETURNS bool AS '
|
||||
<term><replaceable class="Parameter">$TG_when</replaceable></term>
|
||||
<listitem>
|
||||
<para>
|
||||
The string BEFORE or AFTER depending on the event of the trigger call.
|
||||
The string <literal>BEFORE</> or <literal>AFTER</> depending on the
|
||||
type of trigger call.
|
||||
</para>
|
||||
</listitem>
|
||||
</varlistentry>
|
||||
@ -230,7 +525,8 @@ CREATE FUNCTION overpaid_2 (EMP) RETURNS bool AS '
|
||||
<term><replaceable class="Parameter">$TG_level</replaceable></term>
|
||||
<listitem>
|
||||
<para>
|
||||
The string ROW or STATEMENT depending on the event of the trigger call.
|
||||
The string <literal>ROW</> or <literal>STATEMENT</> depending on the
|
||||
type of trigger call.
|
||||
</para>
|
||||
</listitem>
|
||||
</varlistentry>
|
||||
@ -239,8 +535,8 @@ CREATE FUNCTION overpaid_2 (EMP) RETURNS bool AS '
|
||||
<term><replaceable class="Parameter">$TG_op</replaceable></term>
|
||||
<listitem>
|
||||
<para>
|
||||
The string INSERT, UPDATE or DELETE depending on the event of the
|
||||
trigger call.
|
||||
The string <literal>INSERT</>, <literal>UPDATE</> or
|
||||
<literal>DELETE</> depending on the type of trigger call.
|
||||
</para>
|
||||
</listitem>
|
||||
</varlistentry>
|
||||
@ -249,8 +545,9 @@ CREATE FUNCTION overpaid_2 (EMP) RETURNS bool AS '
|
||||
<term><replaceable class="Parameter">$NEW</replaceable></term>
|
||||
<listitem>
|
||||
<para>
|
||||
An array containing the values of the new table row on INSERT/UPDATE
|
||||
actions, or empty on DELETE.
|
||||
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>
|
||||
@ -259,17 +556,9 @@ CREATE FUNCTION overpaid_2 (EMP) RETURNS bool AS '
|
||||
<term><replaceable class="Parameter">$OLD</replaceable></term>
|
||||
<listitem>
|
||||
<para>
|
||||
An array containing the values of the old table row on UPDATE/DELETE
|
||||
actions, or empty on INSERT.
|
||||
</para>
|
||||
</listitem>
|
||||
</varlistentry>
|
||||
|
||||
<varlistentry>
|
||||
<term><replaceable class="Parameter">$GD</replaceable></term>
|
||||
<listitem>
|
||||
<para>
|
||||
The global status data array as described above.
|
||||
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>
|
||||
@ -279,8 +568,8 @@ CREATE FUNCTION overpaid_2 (EMP) RETURNS bool AS '
|
||||
<listitem>
|
||||
<para>
|
||||
A Tcl list of the arguments to the procedure as given in the
|
||||
CREATE TRIGGER statement. The arguments are also accessible as $1 ... $n
|
||||
in the procedure body.
|
||||
CREATE TRIGGER statement. These arguments are also accessible as
|
||||
<literal>$1</literal> ... <literal>$n</literal> in the procedure body.
|
||||
</para>
|
||||
</listitem>
|
||||
</varlistentry>
|
||||
@ -289,14 +578,16 @@ CREATE FUNCTION overpaid_2 (EMP) RETURNS bool AS '
|
||||
</para>
|
||||
|
||||
<para>
|
||||
The return value from a trigger procedure is one of the strings OK or SKIP,
|
||||
or a list as returned by the 'array get' Tcl command. If the return value
|
||||
is OK, the normal operation (INSERT/UPDATE/DELETE) that fired this trigger
|
||||
will take place. Obviously, SKIP tells the trigger manager to silently
|
||||
suppress the operation. The list from 'array get' tells PL/Tcl
|
||||
to return a modified row to the trigger manager that will be inserted instead
|
||||
of the one given in $NEW (INSERT/UPDATE only). Needless to say that all
|
||||
this is only meaningful when the trigger is BEFORE and FOR EACH ROW.
|
||||
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
|
||||
@ -321,222 +612,74 @@ CREATE FUNCTION trigfunc_modcount() RETURNS OPAQUE AS '
|
||||
return [array get NEW]
|
||||
' LANGUAGE 'pltcl';
|
||||
|
||||
CREATE TABLE mytab (num int4, modcnt int4, description text);
|
||||
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>Database Access from PL/Tcl</title>
|
||||
|
||||
<para>
|
||||
The following commands are available to access the database from
|
||||
the body of a PL/Tcl procedure:
|
||||
</para>
|
||||
|
||||
<variablelist>
|
||||
|
||||
<varlistentry>
|
||||
<indexterm>
|
||||
<primary>elog</primary>
|
||||
</indexterm>
|
||||
<term><function>elog</> <replaceable>level</replaceable> <replaceable>msg</replaceable></term>
|
||||
<listitem>
|
||||
<para>
|
||||
Fire a log message. Possible levels are NOTICE, ERROR,
|
||||
FATAL, and DEBUG
|
||||
as for the <function>elog</function> C function.
|
||||
</para>
|
||||
</listitem>
|
||||
</varlistentry>
|
||||
|
||||
<varlistentry>
|
||||
<term><function>quote</> <replaceable>string</replaceable></term>
|
||||
<listitem>
|
||||
<para>
|
||||
Duplicates all occurrences of single quote and backslash characters.
|
||||
It should be used when variables are used in the query string given
|
||||
to <function>spi_exec</function> or
|
||||
<function>spi_prepare</function> (not for the value list on
|
||||
<function>spi_execp</function>).
|
||||
Think about a query string like
|
||||
|
||||
<programlisting>
|
||||
"SELECT '$val' AS ret"
|
||||
</programlisting>
|
||||
|
||||
where the Tcl variable val actually contains <literal>doesn't</literal>. This would result
|
||||
in the final query string
|
||||
|
||||
<programlisting>
|
||||
SELECT 'doesn't' AS ret
|
||||
</programlisting>
|
||||
|
||||
which would cause a parse error during
|
||||
<function>spi_exec</function> or
|
||||
<function>spi_prepare</function>.
|
||||
It should contain
|
||||
|
||||
<programlisting>
|
||||
SELECT 'doesn''t' AS ret
|
||||
</programlisting>
|
||||
|
||||
and has to be written as
|
||||
|
||||
<programlisting>
|
||||
SELECT '[ quote $val ]' AS ret
|
||||
</programlisting>
|
||||
</para>
|
||||
</listitem>
|
||||
</varlistentry>
|
||||
|
||||
<varlistentry>
|
||||
<indexterm>
|
||||
<primary>spi_lastoid</primary>
|
||||
</indexterm>
|
||||
<term><function>spi_lastoid</></term>
|
||||
<listitem>
|
||||
<para>
|
||||
Returns the OID of the last query if it was an INSERT.
|
||||
</para>
|
||||
</listitem>
|
||||
</varlistentry>
|
||||
|
||||
<varlistentry>
|
||||
<term><function>spi_exec</function> <literal>?-count <replaceable>n</replaceable>? ?-array <replaceable>name</replaceable>? <replaceable>query</replaceable> ?<replaceable>loop-body</replaceable>?</literal></term>
|
||||
<listitem>
|
||||
<para>
|
||||
Call parser/planner/optimizer/executor for query.
|
||||
The optional -count value tells <function>spi_exec</function>
|
||||
the maximum number of rows
|
||||
to be processed by the query.
|
||||
</para>
|
||||
<para>
|
||||
If the query is
|
||||
a SELECT statement and the optional loop-body (a body of Tcl commands
|
||||
like in a foreach statement) is given, it is evaluated for each
|
||||
row selected and behaves like expected on continue/break. The values
|
||||
of selected fields are put into variables named as the column names. So a
|
||||
|
||||
<programlisting>
|
||||
spi_exec "SELECT count(*) AS cnt FROM pg_proc"
|
||||
</programlisting>
|
||||
|
||||
will set the variable $cnt to the number of rows in the pg_proc system
|
||||
catalog. If the option -array is given, the column values are stored
|
||||
in the associative array named 'name' indexed by the column name
|
||||
instead of individual variables.
|
||||
|
||||
<programlisting>
|
||||
spi_exec -array C "SELECT * FROM pg_class" {
|
||||
elog DEBUG "have table $C(relname)"
|
||||
}
|
||||
</programlisting>
|
||||
|
||||
will print a DEBUG log message for every row of pg_class. The return value
|
||||
of <function>spi_exec</function> is the number of rows
|
||||
affected by the query as found in
|
||||
the global variable SPI_processed.
|
||||
</para>
|
||||
</listitem>
|
||||
</varlistentry>
|
||||
|
||||
<varlistentry>
|
||||
<term><function>spi_prepare</function> <replaceable>query</replaceable> <replaceable>typelist</replaceable></term>
|
||||
<listitem>
|
||||
<para>
|
||||
Prepares AND SAVES a query plan for later execution. It is a bit different
|
||||
from the C level SPI_prepare in that the plan is automatically copied to the
|
||||
top-level memory context. Thus, there is currently no way of preparing a
|
||||
plan without saving it.
|
||||
</para>
|
||||
<para>
|
||||
If the query references arguments, the type names must be given as a Tcl
|
||||
list. The return value from <function>spi_prepare</function> is a query ID to be used in
|
||||
subsequent calls to <function>spi_execp</function>. See <function>spi_execp</function> for a sample.
|
||||
</para>
|
||||
</listitem>
|
||||
</varlistentry>
|
||||
|
||||
<varlistentry>
|
||||
<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>
|
||||
<listitem>
|
||||
<para>
|
||||
Execute a prepared plan from <function>spi_prepare</> with variable substitution.
|
||||
The optional <literal>-count</literal> value tells <function>spi_execp</> the maximum number of rows
|
||||
to be processed by the query.
|
||||
</para>
|
||||
<para>
|
||||
The optional value for <literal>-nulls</> is a string of spaces and 'n' characters
|
||||
telling <function>spi_execp</function> which of the values are NULL's. If given, it must
|
||||
have exactly the length of the number of values.
|
||||
</para>
|
||||
<para>
|
||||
The <parameter>queryid</> is the ID returned by the <function>spi_prepare</function> call.
|
||||
</para>
|
||||
<para>
|
||||
If there was a <parameter>typelist</> given to <function>spi_prepare</function>, a Tcl list of values of
|
||||
exactly the same length must be given to spi_execp after the query. If
|
||||
the type list on spi_prepare was empty, this argument must be omitted.
|
||||
</para>
|
||||
<para>
|
||||
If the query is a SELECT statement, the same as described for <function>spi_exec</>
|
||||
happens for the loop-body and the variables for the fields selected.
|
||||
</para>
|
||||
<para>
|
||||
Here's an example for a PL/Tcl function using a prepared plan:
|
||||
|
||||
<programlisting>
|
||||
CREATE FUNCTION t1_count(int4, int4) RETURNS int4 AS '
|
||||
if {![ info exists GD(plan) ]} {
|
||||
# prepare the saved plan on the first call
|
||||
set GD(plan) [ spi_prepare \\
|
||||
"SELECT count(*) AS cnt FROM t1 WHERE num >= \\$1 AND num <= \\$2" \\
|
||||
int4 ]
|
||||
}
|
||||
spi_execp -count 1 $GD(plan) [ list $1 $2 ]
|
||||
return $cnt
|
||||
' LANGUAGE 'pltcl';
|
||||
</programlisting>
|
||||
|
||||
Note that each backslash that Tcl should see must be doubled in
|
||||
the query creating the function, since the main parser processes
|
||||
backslashes too on CREATE FUNCTION.
|
||||
Inside the query string given to <function>spi_prepare</> should
|
||||
really be dollar signs to mark the parameter positions and to not let
|
||||
$1 be substituted by the value given in the first function call.
|
||||
</para>
|
||||
</listitem>
|
||||
</varlistentry>
|
||||
|
||||
|
||||
</variablelist>
|
||||
|
||||
</sect2>
|
||||
|
||||
<sect2>
|
||||
<title> Modules and the <function>unknown</> command</title>
|
||||
<para>
|
||||
PL/Tcl has a special support for things often used. It
|
||||
recognizes two magic tables, <literal>pltcl_modules</> and
|
||||
<literal>pltcl_modfuncs</>. If these exist, the module
|
||||
'unknown' is loaded into the interpreter right after
|
||||
creation. Whenever an unknown Tcl procedure is called, the
|
||||
unknown proc is asked to check if the procedure is defined in
|
||||
one of the modules. If this is true, the module is loaded on
|
||||
demand.
|
||||
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>
|
||||
There are support scripts to maintain these tables:
|
||||
<command>pltcl_loadmod</>,<command>pltcl_listmod</>,
|
||||
<command>pltcl_delmod</> and source for the
|
||||
unknown module <filename>share/unknown.pltcl</> that must be loaded
|
||||
into database initially for getting unknown support.
|
||||
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. 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 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>
|
||||
|
||||
Loading…
x
Reference in New Issue
Block a user