ToDo: conditional ALTER TABLE

Hello

I am working on quiet dumps now. i found a small issue.

pg_dump produces a statements

ALTER TABLE ONLY public.b DROP CONSTRAINT b_fk_fkey;
ALTER TABLE ONLY public.a DROP CONSTRAINT a_pkey;

DROP TABLE IF EXISTS public.b;
DROP TABLE IF EXISTS public.a;

Actually there is no a conditional ALTER. These statements must be
before DROPs, but then it can fails when these tables are missing.

So some form like ALTER TABLE IF EXISTS ... should be usefull

Regards

Pavel Stehule