Thread: Plan invalidation

I noticed that the plan invalidation is not immediately effective.
Not sure whether it's worth fixing or has any other side-effects,
but thought would just post it.

I was testing the following scenario:

   session1                            session2
   CREATE TABLE test       (int a, int b);   INSERT INTO TABLE                                SET enable_seqscan = off
                             BEGIN                                PREPARE myplan AS SELECT * FROM TEST
                       WHERE a = 100;                                EXPLAIN EXECUTE myplan; (seq scan)   CREATE INDEX
                                            ----->   EXPLAIN EXECUTE myplan (seq scan)
EXPLAINEXECUTE myplan (index scan)
 

The second "EXPLAIN" in session 2 uses seq scan because the plan
is not invalidated and replanned properly. Ideally it should have
used the index scan.

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. At the end of the command, session 2 calls
CommandCounterIncrement() and gets the invalidation messages.
Hence the next EXPLAIN revalidates the plan properly.

May be this is not such an important issue. But I was wondering
if there are other places in the code where we might miss
or receive invalidation messages with a delay, mostly because
of *lack* of lock conflict ?

Thanks,
Pavan

-- 


EnterpriseDB        http://www.enterprisedb.com

"Pavan Deolasee" <pavan.deolasee@enterprisedb.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
relcacheentry before we try to use it.    * We can skip this in the not-uncommon case that we already had the same    *
typeof lock being requested, since then no one else could have    * modified the relcache entry in an undesirable way.
(Inthe 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

Oh yes, I was wondering about the other parts of the code, not
plan invalidation. Never mind, it was just a thought.


Thanks,
Pavan


--

EnterpriseDB     http://www.enterprisedb.com

"Pavan Deolasee" <pavan.deolasee@gmail.com> writes:
> On 4/3/07, Tom Lane <tgl@sss.pgh.pa.us> wrote:
>> 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.
>> 
> Oh yes, I was wondering about the other parts of the code, not
> plan invalidation. Never mind, it was just a thought.

Well, as that comment notes, we've always had to worry about being sure
that the relcache data structures are up-to-date (or sufficiently
up-to-date, anyway).  I think it's reasonably well debugged.
        regards, tom lane