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Overhaul PL/Tcl documentation.

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REL7_2_STABLE
Tom Lane 24 years ago
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      doc/src/sgml/pltcl.sgml

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@ -1,5 +1,5 @@
<!--
$Header: /cvsroot/pgsql/doc/src/sgml/pltcl.sgml,v 2.17 2001/11/21 05:53:41 thomas Exp $
$Header: /cvsroot/pgsql/doc/src/sgml/pltcl.sgml,v 2.18 2002/01/23 21:08:17 tgl Exp $
-->
<chapter id="pltcl">
@ -16,7 +16,7 @@ $Header: /cvsroot/pgsql/doc/src/sgml/pltcl.sgml,v 2.17 2001/11/21 05:53:41 thoma
<para>
PL/Tcl is a loadable procedural language for the
<productname>PostgreSQL</productname> database system
that enables the Tcl language to be used to create functions and
that enables the Tcl language to be used to write functions and
trigger procedures.
</para>
@ -66,7 +66,9 @@ $Header: /cvsroot/pgsql/doc/src/sgml/pltcl.sgml,v 2.17 2001/11/21 05:53:41 thoma
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.
<filename>createlang</filename> script, for example
<literal>createlang pltcl <replaceable>dbname</></literal> or
<literal>createlang pltclu <replaceable>dbname</></literal>.
</para>
</sect1>
@ -76,23 +78,7 @@ $Header: /cvsroot/pgsql/doc/src/sgml/pltcl.sgml,v 2.17 2001/11/21 05:53:41 thoma
<title>Description</title>
<sect2>
<title><productname>PostgreSQL</productname> Functions and 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. 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>PostgreSQL</productname>
function are different for Tcl too.
</para>
</sect2>
<sect2>
<title>Defining Functions in PL/Tcl</title>
<title>PL/Tcl Functions and Arguments</title>
<para>
To create a function in the PL/Tcl language, use the standard syntax
@ -103,32 +89,68 @@ CREATE FUNCTION <replaceable>funcname</replaceable> (<replaceable>argument-types
' LANGUAGE 'pltcl';
</programlisting>
When the function is called, the arguments are given as
PL/TclU is the same, except that the language should be specified as
<literal>'pltclu'</>.
</para>
<para>
The body of the function is simply a piece of Tcl script.
When the function is called, the argument values are passed as
variables <literal>$1</literal> ... <literal>$n</literal> to the
Tcl procedure body. The result is returned
Tcl script. 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:
returning the greater of two integer values could be defined as:
<programlisting>
CREATE FUNCTION tcl_max (int4, int4) RETURNS int4 AS '
CREATE FUNCTION tcl_max (integer, integer) RETURNS integer AS '
if {$1 > $2} {return $1}
return $2
' LANGUAGE 'pltcl' WITH (isStrict);
</programlisting>
Note the clause <literal>WITH (isStrict)</>, which saves us from
having to think about NULL input values: if a NULL is passed, the
function will not be called at all, but will just return a NULL
result automatically.
</para>
<para>
In a non-strict function,
if the actual value of an argument is NULL, the corresponding
<literal>$n</literal> variable will be set to an empty string.
To detect whether a particular argument is NULL, use the function
<literal>argisnull</>. For example, suppose that we wanted tcl_max
with one null and one non-null argument to return the non-null
argument, rather than NULL:
<programlisting>
CREATE FUNCTION tcl_max (integer, integer) RETURNS integer AS '
if {[argisnull 1]} {
if {[argisnull 2]} { return_null }
return $2
}
if {[argisnull 2]} { return $1 }
if {$1 > $2} {return $1}
return $2
' LANGUAGE 'pltcl';
</programlisting>
</para>
To return a NULL value from a PL/Tcl function, execute
<literal>return_null</literal>.
<para>
As shown above,
to return a NULL value from a PL/Tcl function, execute
<literal>return_null</literal>. This can be done whether the
function is strict or not.
</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
Composite-type arguments are passed to the procedure as Tcl arrays.
The element names of the array are the attribute names of the composite
type. If an attribute in the passed 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>PostgreSQL</productname> documentation) in PL/Tcl
older <productname>PostgreSQL</productname> documentation) in PL/Tcl:
<programlisting>
CREATE FUNCTION overpaid_2 (EMP) RETURNS bool AS '
@ -143,27 +165,298 @@ CREATE FUNCTION overpaid_2 (EMP) RETURNS bool AS '
</programlisting>
</para>
<para>
There is not currently any support for returning a composite-type
result value.
</para>
</sect2>
<sect2>
<title>Data Values in PL/Tcl</title>
<para>
The argument values supplied to a PL/Tcl function's script are simply
the input arguments converted to text form (just as if they had been
displayed by a SELECT statement). Conversely, the <literal>return</>
command will accept any string that is acceptable input format for
the function's declared return type. So, the PL/Tcl programmer can
manipulate data values as if they were just text.
</para>
</sect2>
<sect2>
<title>Global Data in PL/Tcl</title>
<para>
Sometimes (especially when using the SPI functions described later) it
Sometimes it
is useful to have some global status data that is held between two
calls to a procedure. This is easily done since
calls to a procedure or is shared between different procedures.
This is easily done since
all PL/Tcl procedures executed in one backend share the same
safe Tcl interpreter.
safe Tcl interpreter. So, any global Tcl variable is accessible to
all PL/Tcl procedure calls, and will persist for the duration of the
SQL client connection. (Note that PL/TclU functions likewise share
global data, but they are in a different Tcl interpreter and cannot
communicate with PL/Tcl functions.)
</para>
<para>
To help protect PL/Tcl procedures from unwanted side effects,
an array is made available to each procedure via the <function>upvar</>
To help protect PL/Tcl procedures from unintentionally interfering
with each other, a global
array is made available to each procedure via the <function>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
name and the local name is <literal>GD</>. It is recommended that
<literal>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>
<para>
An example of using <literal>GD</> appears in the
<function>spi_execp</function> example below.
</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><function>spi_exec</function> <literal>?-count <replaceable>n</replaceable>? ?-array <replaceable>name</replaceable>? <replaceable>query</replaceable> ?<replaceable>loop-body</replaceable>?</literal></term>
<listitem>
<para>
Execute an SQL query given as a string. An error in the query
causes an error to be raised. Otherwise, the command's return value
is the number of rows processed (selected, inserted, updated, or
deleted) by the query, or zero if the query is a utility
statement. In addition, if the query is a SELECT statement, the
values of the selected columns are placed in Tcl variables as
described below.
</para>
<para>
The optional <literal>-count</> value tells
<function>spi_exec</function> the maximum number of rows
to process in the query. The effect of this is comparable to
setting up the query as a cursor and then saying <literal>FETCH n</>.
</para>
<para>
If the query is a SELECT statement, the values of the SELECT's
result columns are placed into Tcl variables named after the columns.
If the <literal>-array</> option is given, the column values are
instead stored into the named associative array, with the SELECT
column names used as array indexes.
</para>
<para>
If the query is a SELECT statement and no <replaceable>loop-body</>
script is given, then only the first row of results are stored into
Tcl variables; remaining rows, if any, are ignored. No store occurs
if the
SELECT returns no rows (this case can be detected by checking the
result of <function>spi_exec</function>). For example,
<programlisting>
spi_exec "SELECT count(*) AS cnt FROM pg_proc"
</programlisting>
will set the Tcl variable <literal>$cnt</> to the number of rows in
the pg_proc system catalog.
</para>
<para>
If the optional <replaceable>loop-body</> argument is given, it is
a piece of Tcl script that is executed once for each row in the
SELECT result (note: <replaceable>loop-body</> is ignored if the given
query is not a SELECT). The values of the current row's fields
are stored into Tcl variables before each iteration. For example,
<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. This
feature works similarly to other Tcl looping constructs; in
particular <literal>continue</> and <literal>break</> work in the
usual way inside the loop body.
</para>
<para>
If a field of a SELECT result is NULL, the target
variable for it is <quote>unset</> rather than being set.
</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. The saved plan
will be retained for the life of the current backend.
</para>
<para>
The query may use <firstterm>arguments</>, which are placeholders for
values to be supplied whenever the plan is actually executed.
In the query string, refer to arguments
by the symbols <literal>$1</literal> ... <literal>$n</literal>.
If the query uses arguments, the names of the argument types
must be given as a Tcl list. (Write an empty list for
<replaceable>typelist</replaceable> if no arguments are used.)
Presently, the argument types must be identified by the internal
type names shown in pg_type; for example <literal>int4</> not
<literal>integer</>.
</para>
<para>
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 an example.
</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 query previously prepared with <function>spi_prepare</>.
<replaceable>queryid</replaceable> is the ID returned by
<function>spi_prepare</>. If the query references arguments,
a <replaceable>value-list</replaceable> must be supplied: this
is a Tcl list of actual values for the arguments. This must be
the same length as the argument type list previously given to
<function>spi_prepare</>. Omit <replaceable>value-list</replaceable>
if the query has no arguments.
</para>
<para>
The optional value for <literal>-nulls</> is a string of spaces and
<literal>'n'</> characters telling <function>spi_execp</function>
which of the arguments are NULLs. If given, it must have exactly the
same length as the <replaceable>value-list</replaceable>. If it
is not given, all the argument values are non-NULL.
</para>
<para>
Except for the way in which the query and its arguments are specified,
<function>spi_execp</> works just like <function>spi_exec</>.
The <literal>-count</>, <literal>-array</>, and
<replaceable>loop-body</replaceable> options are the same,
and so is the result value.
</para>
<para>
Here's an example of a PL/Tcl function using a prepared plan:
<programlisting>
CREATE FUNCTION t1_count(integer, integer) RETURNS integer 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" \\
[ list int4 int4 ] ]
}
spi_execp -count 1 $GD(plan) [ list $1 $2 ]
return $cnt
' LANGUAGE 'pltcl';
</programlisting>
Note that each backslash that Tcl should see must be doubled when
we type in the function, since the main parser processes
backslashes too in CREATE FUNCTION. We need backslashes inside
the query string given to <function>spi_prepare</> to ensure that
the <literal>$n</> markers will be passed through to
<function>spi_prepare</> as-is, and not
replaced by Tcl variable substitution.
</para>
</listitem>
</varlistentry>
<varlistentry>
<indexterm>
<primary>spi_lastoid</primary>
</indexterm>
<term><function>spi_lastoid</></term>
<listitem>
<para>
Returns the OID of the row inserted by the last
<function>spi_exec</>'d or <function>spi_execp</>'d query,
if that query was a single-row INSERT. (If not, you get zero.)
</para>
</listitem>
</varlistentry>
<varlistentry>
<term><function>quote</> <replaceable>string</replaceable></term>
<listitem>
<para>
Duplicates all occurrences of single quote and backslash characters
in the given string. This may be used to safely quote strings
that are to be inserted into SQL queries given
to <function>spi_exec</function> or
<function>spi_prepare</function>.
For example, 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>.
The submitted query should contain
<programlisting>
SELECT 'doesn''t' AS ret
</programlisting>
which can be formed in PL/Tcl as
<programlisting>
"SELECT '[ quote $val ]' AS ret"
</programlisting>
One advantage of <function>spi_execp</function> is that you don't
have to quote argument values like this, since the arguments are never
parsed as part of an SQL query string.
</para>
</listitem>
</varlistentry>
<varlistentry>
<indexterm>
<primary>elog</primary>
</indexterm>
<term><function>elog</> <replaceable>level</replaceable> <replaceable>msg</replaceable></term>
<listitem>
<para>
Emit a log or error message. Possible levels are <literal>DEBUG</>,
<literal>NOTICE</>, <literal>ERROR</>, and <literal>FATAL</>.
<literal>DEBUG</> and <literal>NOTICE</> simply emit the given message
into the postmaster log (and send it to the client too, in the case of
<literal>NOTICE</>). <literal>ERROR</> raises an error condition:
further execution of the function is abandoned, and the current
transaction is aborted. <literal>FATAL</> aborts the transaction and
causes the current backend to shut down (there is probably no good
reason to use this error level in PL/Tcl functions, but it's provided
for completeness).
</para>
</listitem>
</varlistentry>
</variablelist>
</sect2>
<sect2>
@ -175,13 +468,13 @@ CREATE FUNCTION overpaid_2 (EMP) RETURNS bool AS '
</indexterm>
<para>
Trigger procedures are defined in <productname>PostgreSQL</productname>
as functions without
arguments and a return type of opaque. And so are they in the PL/Tcl
language.
Trigger procedures can be written in PL/Tcl. As is customary in
<productname>PostgreSQL</productname>, a procedure that's to be called
as a trigger must be declared as a function with no arguments
and a return type of <literal>opaque</>.
</para>
<para>
The information from the trigger manager is given to the procedure body
The information from the trigger manager is passed to the procedure body
in the following variables:
<variablelist>
@ -209,10 +502,11 @@ CREATE FUNCTION overpaid_2 (EMP) RETURNS bool AS '
<term><replaceable class="Parameter">$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>
<title> Modules and the <function>unknown</> command</title>
<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.
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>
The <parameter>queryid</> is the ID returned by the <function>spi_prepare</function> call.
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>
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.
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>
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.
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>
<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 <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.
</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.
</para>
<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>

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