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

Hi,

On 2020-10-16 10:42:43 +0300, e(dot)sokolova(at)postgrespro(dot)ru wrote:
> 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!

Interesting idea!

I'm a bit worried that further increasing the size of struct
Instrumentation will increase the overhead of EXPLAIN ANALYZE further -
in some workloads we spend a fair bit of time in code handling that. It
would be good to try to find a few bad cases, and see what the overhead is.

Unfortunately your patch is pretty hard to look at - you seem to have
included your incremental hacking efforts?

> From 7871ac1afe7837a6dc0676a6c9819cc68a5c0f07 Mon Sep 17 00:00:00 2001
> From: "e.sokolova" <e(dot)sokolova(at)postgrespro(dot)ru>
> Date: Fri, 4 Sep 2020 18:00:47 +0300
> Subject: Add min and max statistics without case of
> parallel workers. Tags: commitfest_hotfix.

> From ebdfe117e4074d268e3e7c480b98d375d1d6f62b Mon Sep 17 00:00:00 2001
> From: "e.sokolova" <e(dot)sokolova(at)postgrespro(dot)ru>
> Date: Fri, 11 Sep 2020 23:04:34 +0300
> Subject: Add case of parallel workers. Tags:
> commitfest_hotfix.

> From ecbf04d519e17b8968103364e89169ab965b41d7 Mon Sep 17 00:00:00 2001
> From: "e.sokolova" <e(dot)sokolova(at)postgrespro(dot)ru>
> Date: Fri, 18 Sep 2020 13:35:19 +0300
> Subject: Fix bugs. Tags: commitfest_hotfix.

> From 7566a98bbc33a24052e1334b0afe2cf341c0818f Mon Sep 17 00:00:00 2001
> From: "e.sokolova" <e(dot)sokolova(at)postgrespro(dot)ru>
> Date: Fri, 25 Sep 2020 20:09:22 +0300
> Subject: Fix tests. Tags: commitfest_hotfix.

Greetings,

Andres Freund