Trigger functions can be written in PL/Tcl. PostgreSQL requires that a function 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 function body in the following variables:

The return value from a trigger function can be one of the strings OK or SKIP, or a list of column name/value pairs. 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; the contents of the modified row are specified by the column names and values in the list. Any columns not mentioned in the list are set to null. Returning a modified row is only meaningful for row-level BEFORE INSERT or UPDATE triggers, for which the modified row will be inserted instead of the one given in $NEW; or for row-level INSTEAD OF INSERT or UPDATE triggers where the returned row is used as the source data for INSERT RETURNING or UPDATE RETURNING clauses. In row-level BEFORE DELETE or INSTEAD OF DELETE triggers, returning a modified row has the same effect as returning OK, that is the operation proceeds. The trigger return value is ignored for all other types of triggers.

Here's a little example trigger function 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 FUNCTION trigfunc_modcount('modcnt');

Notice that the trigger function itself does not know the column name; that's supplied from the trigger arguments. This lets the trigger function be reused with different tables.