Re: [PATCH] Add extra statistics to explain for Nested Loop

so 17. 10. 2020 v 0:11 odesílatel Anastasia Lubennikova <
a(dot)lubennikova(at)postgrespro(dot)ru> napsal:

> On 16.10.2020 12:07, Julien Rouhaud wrote:
>
> Le ven. 16 oct. 2020 à 16:12, Pavel Stehule <pavel(dot)stehule(at)gmail(dot)com> a
> écrit :
>
>>
>>
>> pá 16. 10. 2020 v 9:43 odesílatel <e(dot)sokolova(at)postgrespro(dot)ru> napsal:
>>
>>> Hi, hackers.
>>> For some distributions of data in tables, different loops in nested loop
>>> joins can take different time and process different amounts of entries.
>>> It makes average statistics returned by explain analyze not very useful
>>> for DBA.
>>> To fix it, here is the patch that add printing of min and max statistics
>>> for time and rows across all loops in Nested Loop to EXPLAIN ANALYSE.
>>> Please don't hesitate to share any thoughts on this topic!
>>>
>>
>> +1
>>
>> This is great feature - sometimes it can be pretty messy current limited
>> format
>>
>
> +1, this can be very handy!
>
>> Cool.
> I have added your patch to the commitfest, so it won't get lost.
> https://commitfest.postgresql.org/30/2765/
>
> I will review the code next week. Unfortunately, I cannot give any
> feedback about usability of this feature.
>
> User visible change is:
>
> - -> Nested Loop (actual rows=N loops=N)
> + -> Nested Loop (actual min_rows=0 rows=0 max_rows=0
> loops=2)
>

This interface is ok - there is not too much space for creativity. I can
imagine displaying variance or average - but I am afraid about very bad
performance impacts.

Regards

Pavel

>
> Pavel, Julien, could you please say if it looks good?
>
> --
> Anastasia Lubennikova
> Postgres Professional: http://www.postgrespro.com
> The Russian Postgres Company
>
>