Re: Explain buffers wrong counter with parallel plans

> On Jul 27, 2018, at 8:31 AM, Amit Kapila <amit.kapila16@gmail.com> wrote:
> 
> On Thu, Jul 26, 2018 at 7:31 PM, Jonathan S. Katz
> <jonathan.katz@excoventures.com> wrote:
>> 
>>> On Jul 25, 2018, at 10:25 PM, Amit Kapila <amit.kapila16@gmail.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