PL/Tcl is a loadable procedural language for the Postgres database system that enables the Tcl language to be used to create functions and trigger-procedures.
This package was originally written by Jan Wieck.
PL/Tcl offers most of the capabilities a function writer has in the C language, except for some restrictions.
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 over SPI and to raise messages via elog(). There is no way to access internals of the database backend or gaining OS-level access under the permissions of the Postgres user ID like in C. Thus, any unprivileged database user may be permitted to use this language.
The other, internal given, restriction is, that Tcl procedures cannot be used to create input-/output-functions for new data types.
The shared object for the PL/Tcl call handler is automatically built and installed in the Postgres library directory if the Tcl/Tk support is specified in the configuration step of the installation procedure.
In Postgres, 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 procedures pg_proc row as part of their name. Thus, different argtype versions of the same Postgres function are different for Tcl too.
To create a function in the PL/Tcl language, use the known syntax
CREATE FUNCTION funcname (argument-types) RETURNS returntype AS '
# PL/Tcl function body
' LANGUAGE 'pltcl';
When calling this function in a query, the arguments are
given as variables $1 ... $n to the Tcl procedure body. So a little
max function returning the higher of two int4 values would be
created as:
CREATE FUNCTION tcl_max (int4, int4) RETURNS int4 AS '
if {$1 > $2} {return $1}
return $2
' LANGUAGE 'pltcl';
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 Postgres documentation) in PL/Tcl
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';
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. All PL/Tcl procedures executed in one backend share the same safe Tcl interpreter. To help protecting PL/Tcl procedures from side effects, an array is made available to each procedure via the upvar command. The global name of this variable is the procedures internal name and the local name is GD.
Trigger procedures are defined in Postgres as functions without arguments and a return type of opaque. And so are they in the PL/Tcl language.
The informations from the trigger manager are given to the procedure body in the following variables:
The name of the trigger from the CREATE TRIGGER statement.
The object ID of the table that caused the trigger procedure to be invoked.
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.
The string BEFORE or AFTER depending on the event of the trigger call.
The string ROW or STATEMENT depending on the event of the trigger call.
The string INSERT, UPDATE or DELETE depending on the event of the trigger call.
An array containing the values of the new table row on INSERT/UPDATE actions, or empty on DELETE.
An array containing the values of the old table row on UPDATE/DELETE actions, or empty on INSERT.
The global status data array as described above.
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.
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.
Here's a little example trigger procedure that forces an integer value in a table to keep track of the # of updates that are performed on the row. For new row's inserted, the value is initialized to 0 and then incremented on every update operation:
CREATE FUNCTION trigfunc_modcount() RETURNS OPAQUE AS '
switch $TG_op {
INSERT {
set NEW($1) 0
}
UPDATE {
set NEW($1) $OLD($1)
incr NEW($1)
}
default {
return OK
}
}
return [array get NEW]
' LANGUAGE 'pltcl';
CREATE TABLE mytab (num int4, modcnt int4, desc text);
CREATE TRIGGER trig_mytab_modcount BEFORE INSERT OR UPDATE ON mytab
FOR EACH ROW EXECUTE PROCEDURE trigfunc_modcount('modcnt');
The following commands are available to access the database from the body of a PL/Tcl procedure:
Fire a log message. Possible levels are NOTICE, WARN, ERROR, FATAL, DEBUG and NOIND like for the elog() C function.
Duplicates all occurences of single quote and backslash characters. It should be used when variables are used in the query string given to spi_exec or spi_prepare (not for the value list on spi_execp). Think about a query string like
"SELECT '$val' AS ret"
where the Tcl variable val actually contains "doesn't". This
would result in the final query string
"SELECT 'doesn't' AS ret"
what would cause a parse error during spi_exec or
spi_prepare. It should contain
"SELECT 'doesn''t' AS ret"
and has to be written as
"SELECT '[ quote $val ]' AS ret"
Call parser/planner/optimizer/executor for query. The optional -count value tells spi_exec the maximum number of rows to be processed by the query.
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
spi_exec "SELECT count(*) AS cnt FROM pg_proc"
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.
spi_exec -array C "SELECT * FROM pg_class" {
elog DEBUG "have table $C(relname)"
}
will print a DEBUG log message for every row of pg_class. The
return value of spi_exec is the number of rows affected by query as
found in the global variable SPI_processed.
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.
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.
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.
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.
The queryid is the ID returned by the spi_prepare call.
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.
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.
Here's an example for a PL/Tcl function using a prepared plan:
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';
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.
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.
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.