## 37.6. 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. (Empty list elements also appear in the positions of columns that have been dropped, so that the attribute numbering is correct for columns to their right.)

`$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.