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Description

Postgres Functions and Tcl Procedure Names

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.

Defining Functions in PL/Tcl

To create a function in the PL/Tcl language, use the known syntax

CREATE FUNCTION funcname argument-types) RETURNS return-type 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';
     

Global Data in PL/Tcl

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 in PL/Tcl

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:

$TG_name

The name of the trigger from the CREATE TRIGGER statement.

$TG_relid

The object ID of the table that caused the trigger procedure to be invoked.

$TG_relatts

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.

$TG_when

The string BEFORE or AFTER depending on the event of the trigger call.

$TG_level

The string ROW or STATEMENT depending on the event of the trigger call.

$TG_op

The string INSERT, UPDATE or DELETE depending on the event of the trigger call.

$NEW

An array containing the values of the new table row on INSERT/UPDATE actions, or empty on DELETE.

$OLD

An array containing the values of the old table row on UPDATE/DELETE actions, or empty on INSERT.

$GD

The global status data array as described above.

$args

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');
     

Database Access from PL/Tcl

The following commands are available to access the database from the body of a PL/Tcl procedure:

elog level msg

Fire a log message. Possible levels are NOTICE, WARN, ERROR, FATAL, DEBUG and NOIND like for the elog C function.

quote string

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"
        
spi_exec ?-count n? ?-array name? query ?loop-body?

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.
spi_prepare query typelist

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.

spi_exec ?-count n? ?-arrayname? ?-nullsstring? query ?value-list? ?loop-body?

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.
Modules and the unknown command

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.

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