Re: [HACKERS] Performance degradation in Bitmapscan (commit 75ae538bc3168bf44475240d4e0487ee2f3bb376)

On Thu, Dec 8, 2016 at 5:23 PM, Robert Haas <robertmhaas@gmail.com> wrote:
> On Wed, Nov 23, 2016 at 3:33 AM, Andres Freund <andres@anarazel.de> wrote:
>> On 2016-11-18 08:00:40 -0500, Robert Haas wrote:
>>> On Tue, Nov 15, 2016 at 2:28 PM, Andres Freund <andres@anarazel.de> wrote:
>>> > I've a working fix for this, and for a similar issue Robert found. I'm
>>> > still playing around with it, but basically the fix is to make the
>>> > growth policy a bit more adaptive.
>>>
>>> Any chance you can post a patch soon?
>>
>> Here's my WIP series addressing this and related problems. With this
>> we're again noticeably faster than the dynahash implementation, in both
>> the case here, and the query you brought up over IM.
>>
>> This definitely needs some more TLC, but the general approach seems
>> good. I particularly like that it apparently allows us to increase the
>> default fillfactor without much downside according to my measurements.
>
> Are you going to commit something here?  At least enough to make
> Finalize HashAgg -> Gather -> Partial HashAgg terminate in finite
> time?  Because the fact that it doesn't really sucks.

I took a look at Andres's patches today and saw that they can't really
be applied as-is, because they "temporarily" change the master's
ParallelWorkerNumber but have no provision for restoring it after an
ERROR.  I think that approach isn't right anyway; it seems to me that
what we want to do is set hash_iv based on we're in a Partial HashAgg,
not whether we're in a parallel worker.  For instance, if we're
somehow in a nodeSubplan.c there's no need for this just because we
happen to be in a worker -- I think.  That led me to develop the
attached patch.

This may not be perfect, but it causes TPC-H Q18 to complete instead
of hanging forever, so I'm going to commit it RSN unless there are
loud objections combined with promising steps toward some alternative
fix.  It's been over a month since these problems were reported, and
it is not good that the tree has been in a broken state for that
entire time.

-- 
Robert Haas
EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company

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