PostgreSQL 7.4 Documentation
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# 38.6. Trigger Procedures in PL/Tcl

Trigger procedures can be written in PL/Tcl. PostgreSQL requires that a procedure that is to be called as a trigger must be declared as a function with no arguments and a return type of trigger.

The information from the trigger manager is passed 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 table column names, prefixed with an empty list element. So looking up a column name in the list with Tcl's `lsearch` command returns the element's number starting with 1 for the first column, the same way the columns are customarily numbered in PostgreSQL.

$TG_when

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

$TG_level

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

$TG_op

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

$NEW

  An associative array containing the values of the new table row for `INSERT` or `UPDATE` actions, or empty for `DELETE`. The array is indexed by column name. Columns that are null will not appear in the array.

$OLD

  An associative array containing the values of the old table row for `UPDATE` or `DELETE` actions, or empty for `INSERT`. The array is indexed by column name. Columns that are null will not appear in the array.

$args

  A Tcl list of the arguments to the procedure as given in the `CREATE TRIGGER` statement. These arguments are also accessible as $1 ... $n in the procedure body.

The return value from a trigger procedure can be 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 operation (`INSERT`/`UPDATE`/`DELETE`) that fired the trigger will proceed normally. 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` and `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.

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.

    CREATE FUNCTION trigfunc_modcount() RETURNS trigger 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 integer, description text, modcnt integer);

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

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 reused with different tables.

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