[Bizgres-general] Another use case

[Gavin Sherry]

On Fri, 11 Aug 2006, Mark Kirkwood wrote:

> Mark Kirkwood wrote:
> >
> > Hmm , Yeah - good scenario. We might be ok if it is only the
> > [Row]Exclusive locks on the target relation that I'm dropping - as there
> > will still be AccessShare ones on all the relations in the query - which
> > should prevent the DROP COLUMN or similar from beginning.
> >
> >
>
> Actually Gavin is correct, we have to be a bit more subtle than that -
> merely dropping the [Row]ExclusiveLock could leave the target relation
> without any lock at all. However, I think taking an AccessShareLock and
> *then* dropping the [Row]ExclusiveLock should work as intended.

I wonder if there's a deadlock condition where you could have two queries
where query 1 is being demoted from RowExclusive to AccessShare and query
2 is being promoted from AccessShare to RowExclusive...

Actually, it doesn't seem like there is because query 1 will definately
get the AccessShare lock. However, take a good look at some of the
comments in LockAcquire/Release because lock demotion is not something the
code currently expects to happen, it seems.

> A quick piece of hackery to test this idea seemed to leave locks as
> indicated. - we can no doubt discuss this sort of thing in more detail
> next week when I'm at Greenplum (assuming planes from here to US don't
> get grounded.... ).

Hmm... I recommend you ring your airline if you plan to take your laptop
with you. Lots of talk on the news today about not being able to take
laptops aboard US bound planes :-(.

Thanks,

Gavin