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Thread: Check explain plan of a running query in other session

Check explain plan of a running query in other session

From
Jonas Gassenmeyer
Date:
Hello,

I am asking myself for a while if there is an easy option (via dictionary views like pg_stat_activity or similar) to check what execution plan was chosen for a long running SQL statement while it is running in a different session?
I have a performance problem in a plpgsql procedure that executes an update within a loop. My guess is that it chose a generic plan instead of bind peeking and then does not use an index.

I am not able to proof my theory, since I don't know how to get the explain plan for it.
For me the easiest would be to check the running statement in a different session and "hook into" the other plpgsql session to check what the optimizer chose.

Is that something I can do easily? If not: What is the PostgreSQL way of doing it?

Danke und viele Grüße
Jonas

Re: Check explain plan of a running query in other session

From
Laurenz Albe
Date:
On Mon, 2021-11-08 at 09:20 +0100, Jonas Gassenmeyer wrote:
> I am asking myself for a while if there is an easy option (via dictionary views like
> pg_stat_activity or similar) to check what execution plan was chosen for a long
> running SQL statement while it is running in a different session?
> I have a performance problem in a plpgsql procedure that executes an update within
> a loop. My guess is that it chose a generic plan instead of bind peeking and then
> does not use an index.
> 
> I am not able to proof my theory, since I don't know how to get the explain plan for it.
> For me the easiest would be to check the running statement in a different session and
> "hook into" the other plpgsql session to check what the optimizer chose.
> 
> Is that something I can do easily? If not: What is the PostgreSQL way of doing it?

There is nothing like that built into PostgreSQL, but you could try pg_show_plans:
https://github.com/cybertec-postgresql/pg_show_plans

Yours,
Laurenz Albe
-- 
Cybertec | https://www.cybertec-postgresql.com

Re: Check explain plan of a running query in other session

From
Rafia Sabih
Date:
On Mon, 8 Nov 2021 at 09:20, Jonas Gassenmeyer <gassenmj@gmail.com> wrote:
>
> Hello,
>
> I am asking myself for a while if there is an easy option (via dictionary views like pg_stat_activity or similar) to
checkwhat execution plan was chosen for a long running SQL statement while it is running in a different session?
 
> I have a performance problem in a plpgsql procedure that executes an update within a loop. My guess is that it chose
ageneric plan instead of bind peeking and then does not use an index.
 
>
> I am not able to proof my theory, since I don't know how to get the explain plan for it.
> For me the easiest would be to check the running statement in a different session and "hook into" the other plpgsql
sessionto check what the optimizer chose.
 
>
> Is that something I can do easily? If not: What is the PostgreSQL way of doing it?

It is for reasons like these that we at Zalando developed an extension
pg_mon (https://github.com/RafiaSabih/pg_mon),
There you can find details like what are the scan methods and joins
etc used by the query.
You can give this a try.

-- 
Regards,
Rafia Sabih