Re: Plan invalidation

"Pavan Deolasee" <pavan(dot)deolasee(at)enterprisedb(dot)com> writes:
> I traced it a bit and it seems that the invalidation messages
> are not accepted in session 2 because the locks are already held
> on the relation.

Right, because of this coding in LockRelationOid():

/*
* Now that we have the lock, check for invalidation messages, so that we
* will update or flush any stale relcache entry before we try to use it.
* We can skip this in the not-uncommon case that we already had the same
* type of lock being requested, since then no one else could have
* modified the relcache entry in an undesirable way. (In the case where
* our own xact modifies the rel, the relcache update happens via
* CommandCounterIncrement, not here.)
*/
if (res != LOCKACQUIRE_ALREADY_HELD)
AcceptInvalidationMessages();

We could remove the optimization and do AcceptInvalidationMessages
always, but I think that cure would be a great deal worse than the
disease --- it would hugely increase the contention for SInvalLock.

I'm not particularly worried about missing a potential improvement
in the plan during the first command after a change is committed.
If the invalidation were something that *had* to be accounted for,
such as a dropped index, then there should be adequate locking for it;
plancache is not introducing any new bug that wasn't there before.

regards, tom lane