deleting a foreign key that has no references

Hello,

I'm using 8.1.8, and I have a situation where a record in one table is
only meaningful when it is referenced via foreign key by one or more
records in any one of several tables.

So, really what I want is when one of the referring records is deleted,
to have a trigger check to see if it was the last one to use that
foreign key, and if so, to delete that other record, too.

My first implementation of this functionality was to write a trigger
function that executed a COUNT(*) on all of the tables that could have a
reference in them.  That became way too slow for the number of records
in these tables.

Then, I tried to setting ON DELETE RESTRICT or NO ACTION on the foreign
key constraint, and then trying to catch the exception thrown when a
deletion attempt is made on the record.  However, it seems that this
PL/pgsql snippet fails to catch such an error:

  BEGIN EXCEPTION
      WHEN RAISE_EXCEPTION THEN
          RETURN NULL;
      WHEN OTHERS THEN
          RETURN NULL;
  END;

But, really, I just want to be able to test to see how many references
there are to a key.  Is there some way to do that?

Thank you,
Glen Mabey