Re: operator exclusion constraints

On Mon, 2009-11-09 at 18:03 +0000, Greg Stark wrote:
> Out of curiosity, is this feature at all similar to SQL assertions?
> What would we be missing to turn this into them?

I addressed that here:

http://archives.postgresql.org/pgsql-hackers/2009-11/msg00049.php

The exclusion constraint mechanism can enforce a subset of the
constraints that ASSERT can express; although the same goes for all
other constraints, because ASSERT is very general.

The exclusion constraint mechanism requires finding the physical tuples
that cause a conflict, so that we know when to wait and on which
transaction to wait. Otherwise, we have to wait on all transactions;
i.e. serialize.

The problem with ASSERT is that it expresses a constraint based on a
query, which can return arbitrary logical records after an arbitrary
amount of manipulation. So there's no way to work backwards. If we try,
we'll end up either:
(a) supporting only a tiny subset, and throwing bizarre errors that
users don't understand when they try to work outside the template; or
(b) deciding to serialize when we can't do better, and again, users
will be confused about the performance and locking characteristics.

Regards,
Jeff Davis