Am Montag, 9. Februar 2004 06:06 schrieb David Garamond:
> SQL Server only allow one NULL in a unique constraint column (it's the
> unique index that does that, so the unique constraint behaves like that
> too). The question is, what is the best way to simulate that behaviour
> in Postgres? Can a CHECK constraint does that? Will a trigger with
> SELECT count(*) ... WHERE f IS NULL be too slow if the table is large?
You can time that yourself (but I suspect it won't be pretty), but I think
that you're going to have all kinds of other problems if you base your
database schema on the assumption that NULL = NULL. Better fix the
application.