Re: Re: [COMMITTERS] pgsql: Remove O(N^2) performance issue with multiple SAVEPOINTs.

On Thu, Jul 21, 2011 at 5:56 PM, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
> Heikki Linnakangas <heikki(dot)linnakangas(at)enterprisedb(dot)com> writes:
>> Instead of leaving the locks dangling to an already-destroyed resource
>> owner, how about assigning all locks directly to the top-level resource
>> owner in one sweep? That'd still be much better than the old way of
>> recursively reassigning them up the subtransaction tree, one level at a
>> time.
>
> I haven't actually read the patch, but the reason for pushing them up
> only one level at a time is that if an intermediate-level subtransaction
> aborts, the locks taken by its child subtransactions have to be released
> at that time.  It sure sounds like this patch broke that.

The only path altered by the patch was the
final-commit-while-in-a-subxact, so I don't see a problem in the part
you mention.

At commit all the locks get transferred to the parent, so we scan the
the lock table repeatedly, giving O(N^2).

I think I'll just revert it though. Subtransactions need a lot of
tuning but this isn't high enough up my list to be worth the work.

--
 Simon Riggs                   http://www.2ndQuadrant.com/
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