Robert Haas <robertmhaas@gmail.com> writes:
> On Wed, Feb 20, 2019 at 11:03 AM Tom Lane <tgl@sss.pgh.pa.us> wrote:
>> What I was wondering about was the possibility of the set of
>> tables-that-need-to-be-locked-at-all changing. Maybe that won't
>> create an issue either, but I'm not quite sure.
> That's pretty much what I was thinking, too. I think it might be fair
> to say, however, that if it does give rise to deadlock situations,
> they will be corner cases. For instance, suppose you lock are busy
> locking top-down and, meanwhile, somebody detaches a partition you
> haven't gotten around to locking yet and tries to attach it someplace
> higher up in the partition hierarchy. I think that there's a
> more-or-less unavoidable deadlock there. And there may be other cases
> where it is practically avoidable but we will fail to avoid it. But I
> don't think it's such a common scenario that we have two concurrent
> DDL commands on the same partitioning hierarchy that we should stress
> about it too much. If the common cases work OK, a theoretical
> deadlock risk in some more obscure case seems acceptable to me, if it
> means we get a significant performance boost.
I agree that any deadlock would have to involve somebody doing something
quite odd --- not just one partition-oriented operation, but something
taking multiple strong locks without regard to the partition structure.
So I don't see a problem with taking that risk; people doing that sort
of thing are probably at risk of deadlocks no matter what we do here.
Looking at the patch itself, I agree that a bit more attention to comments
is needed, and I wonder whether David has found all the places where
it's now necessary to s/NoLock/RowExclusiveLock/. I don't have any
other objections.
regards, tom lane