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 <chapter id="pltcl">
  <title>PL/Tcl - TCL Procedural Language</title>

  <para>
   PL/Tcl is a loadable procedural language for the
   <productname>Postgres</productname> database system
   that enables the Tcl language to be used to create functions and
   trigger procedures.
  </para>

  <para>
   This package was originally written by Jan Wieck.
  </para>

  <!-- **** PL/Tcl overview **** -->

  <sect1 id="pltcl-overview">
   <title>Overview</title>

   <para>
    PL/Tcl offers most of the capabilities a function
    writer has in the C language, except for some restrictions.
   </para>
   <para>
    The good restriction is that everything is executed in a safe
    Tcl interpreter. In addition to the limited command set of safe Tcl, only
    a few commands are available to access the database via SPI and to raise
    messages via elog(). There is no way to access internals of the
    database backend or to gain OS-level access under the permissions of the
    <productname>Postgres</productname> user ID, as a C function can do.
    Thus, any unprivileged database user may be
    permitted to use this language.
   </para>
   <para>
    The other, implementation restriction is that Tcl procedures cannot
    be used to create input/output functions for new data types.
   </para>
   <para>
    Sometimes it is desirable to write Tcl functions that are not restricted
    to safe Tcl --- for example, one might want a Tcl function that sends
    mail.  To handle these cases, there is a variant of PL/Tcl called PL/TclU
    (for untrusted Tcl).  This is the exact same language except that a full
    Tcl interpreter is used.  <emphasis>If PL/TclU is used, it must be
    installed as an untrusted procedural language</emphasis> so that only
    database superusers can create functions in it.  The writer of a PL/TclU
    function must take care that the function cannot be used to do anything
    unwanted, since it will be able to do anything that could be done by
    a user logged in as the database administrator.
   </para>
   <para>
    The shared object for the PL/Tcl and PL/TclU call handlers is
    automatically built and installed in the
    <productname>Postgres</productname> 
    library directory if Tcl/Tk support is specified
    in the configuration step of the installation procedure.  To install
    PL/Tcl and/or PL/TclU in a particular database, use the
    <filename>createlang</filename> script.
   </para>
  </sect1>

  <!-- **** PL/Tcl description **** -->

  <sect1 id="pltcl-description">
   <title>Description</title>

   <sect2>
    <title><productname>Postgres</productname> Functions and Tcl Procedure Names</title>

    <para>
     In <productname>Postgres</productname>, one and the 
     same function name can be used for
     different functions as long as the number of arguments or their types
     differ. This would collide with Tcl procedure names. To offer the same
     flexibility in PL/Tcl, the internal Tcl procedure names contain the object
     ID of the procedure's pg_proc row as part of their name. Thus, different
     argtype versions of the same <productname>Postgres</productname> 
     function are different for Tcl too.
    </para>

   </sect2>

   <sect2>
    <title>Defining Functions in PL/Tcl</title>

    <para>
     To create a function in the PL/Tcl language, use the standard syntax

     <programlisting>
CREATE FUNCTION <replaceable>funcname</replaceable> (<replaceable>argument-types</replaceable>) RETURNS <replaceable>return-type</replaceable> AS '
    # PL/Tcl function body
' LANGUAGE 'pltcl';
     </programlisting>

     When the function is called, the arguments are given as
     variables <literal>$1</literal> ... <literal>$n</literal> to the
     Tcl procedure body.  The result is returned
     from the Tcl code in the usual way, with a <literal>return</literal>
     statement.  For example, a function
     returning the higher of two int4 values could be defined as:

     <programlisting>
CREATE FUNCTION tcl_max (int4, int4) RETURNS int4 AS '
    if {$1 > $2} {return $1}
    return $2
' LANGUAGE 'pltcl';
     </programlisting>

     To return a NULL value from a PL/Tcl function, execute
     <literal>return_null</literal>.
    </para>

    <para>
     Composite type arguments are given to the procedure as Tcl arrays.
     The element names
     in the array are the attribute names of the composite
     type. If an attribute in the actual row
     has the NULL value, it will not appear in the array! Here is
     an example that defines the overpaid_2 function (as found in the
     older <productname>Postgres</productname> documentation) in PL/Tcl

     <programlisting>
CREATE FUNCTION overpaid_2 (EMP) RETURNS bool AS '
    if {200000.0 < $1(salary)} {
        return "t"
    }
    if {$1(age) < 30 && 100000.0 < $1(salary)} {
        return "t"
    }
    return "f"
' LANGUAGE 'pltcl';
     </programlisting>
    </para>

   </sect2>

   <sect2>
    <title>Global Data in PL/Tcl</title>

    <para>
     Sometimes (especially when using the SPI functions described later) it
     is useful to have some global status data that is held between two
     calls to a procedure.  This is easily done since
     all PL/Tcl procedures executed in one backend share the same
     safe Tcl interpreter.
    </para>
    <para>
     To help protect PL/Tcl procedures from unwanted side effects,
     an array is made available to each procedure via the upvar
     command. The global name of this variable is the procedure's internal
     name and the local name is GD.  It is recommended that GD be used
     for private status data of a procedure.  Use regular Tcl global variables
     only for values that you specifically intend to be shared among multiple
     procedures.
    </para>
   </sect2>

   <sect2>
    <title>Trigger Procedures in PL/Tcl</title>

    <para>
     Trigger procedures are defined in <productname>Postgres</productname>
     as functions without
     arguments and a return type of opaque. And so are they in the PL/Tcl
     language.
    </para>
    <para>
     The information from the trigger manager is given to the procedure body
     in the following variables:

     <variablelist>

      <varlistentry>
       <term><replaceable class="Parameter">$TG_name</replaceable></term>
       <listitem>
	<para>
	 The name of the trigger from the CREATE TRIGGER statement.
	</para>
       </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 tables field names prefixed with an empty list element.
	 So looking up an element name in the list with the lsearch Tcl command
	 returns the same positive number starting from 1 as the fields are numbered
	 in the pg_attribute system catalog.
	</para>
       </listitem>
      </varlistentry>

      <varlistentry>
       <term><replaceable class="Parameter">$TG_when</replaceable></term>
       <listitem>
	<para>
	 The string BEFORE or AFTER depending on the event of the trigger call.
	</para>
       </listitem>
      </varlistentry>

      <varlistentry>
       <term><replaceable class="Parameter">$TG_level</replaceable></term>
       <listitem>
	<para>
	 The string ROW or STATEMENT depending on the event of the trigger call.
	</para>
       </listitem>
      </varlistentry>

      <varlistentry>
       <term><replaceable class="Parameter">$TG_op</replaceable></term>
       <listitem>
	<para>
	 The string INSERT, UPDATE or DELETE depending on the event of the 
	 trigger call.
	</para>
       </listitem>
      </varlistentry>

      <varlistentry>
       <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.
	</para>
       </listitem>
      </varlistentry>

      <varlistentry>
       <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.
	</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. The arguments are also accessible as $1 ... $n
	 in the procedure body.
	</para>
       </listitem>
      </varlistentry>

     </variablelist>
    </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.
    </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 OPAQUE 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 int4, modcnt int4, description text);

CREATE TRIGGER trig_mytab_modcount BEFORE INSERT OR UPDATE ON mytab
    FOR EACH ROW EXECUTE PROCEDURE trigfunc_modcount('modcnt');
     </programlisting>

    </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>
      <term>elog <replaceable>level</replaceable> <replaceable>msg</replaceable></term>
      <listitem>
       <para>
	Fire a log message. Possible levels are NOTICE, ERROR,
	FATAL, DEBUG and NOIND
	as for the <function>elog</function> C function.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term>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 "doesn't". 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>
      <term>spi_exec ?-count <replaceable>n</replaceable>? ?-array <replaceable>name</replaceable>? <replaceable>query</replaceable> ?<replaceable>loop-body</replaceable>?</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>spi_prepare <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
	toplevel 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 spi_prepare is a query ID to be used in
	subsequent calls to spi_execp. See spi_execp for a sample.
       </para>
      </listitem>
     </varlistentry>

     <varlistentry>
      <term>spi_exec ?-count <replaceable>n</replaceable>? ?-array<replaceable>name</replaceable>? ?-nulls<replaceable>string</replaceable>? <replaceable>queryid</replaceable> ?<replaceable>value-list</replaceable>? ?<replaceable>loop-body</replaceable>?</term>
      <listitem>
       <para>
	Execute a prepared plan from spi_prepare with variable substitution.
	The optional -count value tells spi_execp the maximum number of rows
	to be processed by the query.
       </para>
       <para>
	The optional value for -nulls is a string of spaces and 'n' characters
	telling spi_execp which of the values are NULL's. If given, it must
	have exactly the length of the number of values.
       </para>
       <para>
	The queryid is the ID returned by the spi_prepare call.
       </para>
       <para>
	If there was a typelist given to spi_prepare, 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 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 &gt;= \\$1 AND num &lt;= \\$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 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>

     <varlistentry>
      <term>
       Modules and the unknown command
      </term>
      <listitem>
       <para>
	PL/Tcl has a special support for things often used. It recognizes two
	magic tables, pltcl_modules and 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.
	To enable this behavior, the PL/Tcl call handler must be compiled
	with -DPLTCL_UNKNOWN_SUPPORT set.
       </para>
       <para>
	There are support scripts to maintain these tables in the modules
	subdirectory of the PL/Tcl source including the source for the
	unknown module that must get installed initially.
       </para>
      </listitem>
     </varlistentry>

    </variablelist>

   </sect2>

  </sect1>
 </chapter>

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