Re: Explain buffers wrong counter with parallel plans

> On Jul 27, 2018, at 8:31 AM, Amit Kapila <amit(dot)kapila16(at)gmail(dot)com> wrote:
>
> On Thu, Jul 26, 2018 at 7:31 PM, Jonathan S. Katz
> <jonathan(dot)katz(at)excoventures(dot)com> wrote:
>>
>>> On Jul 25, 2018, at 10:25 PM, Amit Kapila <amit(dot)kapila16(at)gmail(dot)com> wrote:
>>>
>>>
>>> You mean to say the number (Buffers: shared read=442478) in HEAD,
>>> right? If so, yeah, I am also wondering why the results of the patch
>>> are different in HEAD and 11beta2. So, if I read correctly, the
>>> numbers in 11beta2 appears correct, but on HEAD it is not correct, it
>>> should have divided the buffers read by loops.
>
> I want to correct myself here, the numbers on HEAD are correct, but
> not on PG11beta2. Is there any chance that either you forgot to apply
> the patch or maybe it is not using correct binaries in case of
> 11beta2.

I need to correct myself too as I was unclear - that was an *unpatched*
11beta2, sorry for the confusion.

>>> I will figure that
>>> out, but this is just to clarify that both of us are seeing the
>>> results in the same way.
>>
>> I’ll try it again but patch it against 11beta2 and see what results I get.
>>
>>>
>>>> and there appears to be a performance hit.
>>>>
>>>
>>> Do you mean to say the performance of the same query in 11beta2 and
>>> HEAD or something else?
>>
>> Correct. But per your advice let me try running it against a patched
>> version of 11beta2 and see what happens.
>>
>
> Yeah, that would be better. Today, I have tried the patch on both
> Head and PG11 and I am getting same and correct results.

I have applied the the patch to PG11beta2 and tested. I received
similar numbers to to the patched HEAD tests, e.g:

=> EXPLAIN (analyze,buffers,timing off,costs off) SELECT count(*) FROM t1;
QUERY PLAN
--------------------------------------------------------------------------
Finalize Aggregate (actual rows=1 loops=1)
Buffers: shared hit=224 read=442254
-> Gather (actual rows=7 loops=1)
Workers Planned: 6
Workers Launched: 6
Buffers: shared hit=224 read=442254
-> Partial Aggregate (actual rows=1 loops=7)
Buffers: shared hit=224 read=442254
-> Parallel Seq Scan on t1 (actual rows=14285714 loops=7)
Buffers: shared hit=224 read=442254
Planning Time: 0.104 ms
Execution Time: 5308.140 ms

I ran the tests a few more times and I understand why the execution
times vary (with an explanation from Andres) so I am satisfied.

Thanks for working on this!

Jonathan