Re: ALTER TABLE SET STATISTICS requires AccessExclusiveLock

On Jul 16, 2010, at 6:01 PM, Tom Lane <tgl@sss.pgh.pa.us> wrote:
> Andres Freund <andres@anarazel.de> writes:
>> What could the join removal path (and similar places) *possibly* do against 
>> such a case? Without stopping to use SnapshotNow I dont see any way :-(
> 
> But the planner, along with most of the rest of the backend, *does* use
> SnapshotNow when examining the system catalogs.
> 
> I share your feeling of discomfort but so far I don't see a hole in
> Simon's argument.  Adding a constraint should never make a
> previously-correct plan incorrect.  Removing one is a very different
> story, but he says he's not changing that case.  (Disclaimer: I have
> not read the patch.)

Perhaps we should start by deciding whether Andres' case is a bug in the first place, and then we can argue about whether it's a join-removal bug, a lock-weakening bug, or a preexisting bug.

...Robert