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

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
> <mailto:pavel(dot)stehule(at)gmail(dot)com>> a écrit :
>
>
>
> pá 16. 10. 2020 v 9:43 odesílatel <e(dot)sokolova(at)postgrespro(dot)ru
> <mailto: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)

Pavel, Julien, could you please say if it looks good?

--
Anastasia Lubennikova
Postgres Professional: http://www.postgrespro.com
The Russian Postgres Company