Stephan Szabo <sszabo@megazone23.bigpanda.com> writes:
> On 30 Nov 2002, Neil Conway wrote:
>> Have we decided how this would even work? Last I heard, Tom still had
>> some major reservations about the practicality of implementing these --
>> for example, would you re-evaluate all constraints that SELECT from a
>> table when the table changes?
> You'd have to either do it in all cases or come up with something that was
> smart enough to limit the cases to some extent based on the expression. I
> doubt that it'd perform terribly well, especially at first.
Note that you can get the "stupid" semantics (run the subselect only
when the constrained table changes) today: just hide the subselect in
a user-defined function that's called from the constraint expression.
Or put the whole check in a trigger instead of using a constraint.
I don't think we should bother with direct support of subselects in
constraints unless we can come up with an implementation that is
significantly better than what you can accomplish with these
workarounds.
regards, tom lane