Efficiently delete rows not referenced by a foreign key

Hi All,

I have a database schema where if row is deleted from one table the rows
it references in another table should also be deleted, unless still
referenced by something else.

Eg. Table A has foreign key to table B. When I delete a row from A I
also want to delete the referenced row in B, unless it's still
referenced by something else (which may be another row in A or in a
completely different table C).

The way I currently do this is to have an AFTER DELETE FOR EACH ROW
trigger on A, which attempts to delete the row in B, but catches and
ignores a foreign_key_violation exception. This works (the foreign keys
don't have ON DELETE CASCADE), but it's slow when deleting many rows.

A single query that deletes all the referenced rows in B, if they're not
referenced by A or C, is much faster. The problem with that approach is
it's error-prone to write and maintain. There may be many other tables
that reference B and if they ever change this query has to be updated,
which is a maintenance nightmare. I could try to auto-generate the SQL
for it by finding foreign key constraints referencing B in
information_schema, but that's not a trivial exercise. It also falls
short in a more complicated scenario where I want to delete rows in
multiple tables (A1, A2, ...) that may all reference B.

Is there an easier way to do this? Postgres obviously knows about all
the foreign keys, so is there any way to get it to do the checking for
me? I mean a way that works in bulk, not one row at a time.

Regards,

Evan