44.5. Trigger Functions #

   When a function is used as a trigger, the dictionary TD contains
   trigger-related values:

   TD["event"]
          contains the event as a string: INSERT, UPDATE, DELETE, or
          TRUNCATE.

   TD["when"]
          contains one of BEFORE, AFTER, or INSTEAD OF.

   TD["level"]
          contains ROW or STATEMENT.

   TD["new"]
          TD["old"]
          For a row-level trigger, one or both of these fields contain the
          respective trigger rows, depending on the trigger event.

   TD["name"]
          contains the trigger name.

   TD["table_name"]
          contains the name of the table on which the trigger occurred.

   TD["table_schema"]
          contains the schema of the table on which the trigger occurred.

   TD["relid"]
          contains the OID of the table on which the trigger occurred.

   TD["args"]
          If the CREATE TRIGGER command included arguments, they are
          available in TD["args"][0] to TD["args"][n-1].

   If TD["when"] is BEFORE or INSTEAD OF and TD["level"] is ROW, you can
   return None or "OK" from the Python function to indicate the row is
   unmodified, "SKIP" to abort the event, or if TD["event"] is INSERT or
   UPDATE you can return "MODIFY" to indicate you've modified the new row.
   Otherwise the return value is ignored.