Re: BUG #16109: Postgres planning time is high across version (Exposebuffer usage during planning in EXPLAIN)

On 2020/04/02 3:47, Julien Rouhaud wrote:
> On Wed, Apr 1, 2020 at 7:51 PM Fujii Masao <masao.fujii@oss.nttdata.com> wrote:
>>
>>
>> On 2020/03/31 10:31, Justin Pryzby wrote:
>>> On Wed, Jan 29, 2020 at 12:15:59PM +0100, Julien Rouhaud wrote:
>>>> Rebase due to conflict with 3ec20c7091e97.
>>>
>>> This is failing to apply probably since 4a539a25ebfc48329fd656a95f3c1eb2cda38af3.
>>> Could you rebase?   (Also, not sure if this can be set as RFC?)
>>
>> I updated the patch. Attached.
> 
> Thanks a lot!  I'm sorry I missed Justin's ping, and it I just
> realized that my cron job that used to warn me about cfbot failure was
> broken :(
> 
>> +/* Compute the difference between two BufferUsage */
>> +BufferUsage
>> +ComputeBufferCounters(BufferUsage *start, BufferUsage *stop)
>>
>> Since BufferUsageAccumDiff() was exported, ComputeBufferCounters() is
>> no longer necessary. In the patched version, BufferUsageAccumDiff() is
>> used to calculate the difference of buffer usage.
> 
> Indeed, exposing BufferUsageAccumDiff wa definitely a good thing!
> 
>> +       if (es->summary && (planduration || es->buffers))
>> +               ExplainOpenGroup("Planning", "Planning", true, es);
>>
>> Isn't it more appropriate to check "bufusage" instead of "es->buffers" here?
>> The patch changes the code so that "bufusage" is checked.
> 
> AFAICS not unless ExplainOneQuery is also changed to pass a NULL
> pointer if the BUFFER option wasn't provided (and maybe also
> optionally skip the planning buffer computation).  With this version
> you now get:
> 
> =# explain (analyze, buffers off) update t1 set id = id;
>                                                QUERY PLAN
> -------------------------------------------------------------------------------------------------------
>   Update on t1  (cost=0.00..22.70 rows=1270 width=42) (actual
> time=0.170..0.170 rows=0 loops=1)
>     ->  Seq Scan on t1  (cost=0.00..22.70 rows=1270 width=42) (actual
> time=0.050..0.054 rows=1 loops=1)
>   Planning Time: 1.461 ms
>     Buffers: shared hit=25
>   Execution Time: 1.071 ms
> (5 rows)
> 
> which seems wrong to me.
> 
> I reused the es->buffers to avoid having needing something like:

Yes, you're right! So I updated the patch as you suggested.
Attached is the updated version of the patch.
Thanks for the review!

Regards,

-- 
Fujii Masao
Advanced Computing Technology Center
Research and Development Headquarters
NTT DATA CORPORATION