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 -- Sent via pgsql-hackers mailing list (pgsql-hackers@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-hackers
