Re: RFC: Logging plan of the running query

On Wed, 12 May 2021 at 13:24, torikoshia <torikoshia(at)oss(dot)nttdata(dot)com> wrote:
>
> Hi,
>
> During the discussion about memory contexts dumping[1], there
> was a comment that exposing not only memory contexts but also
> query plans and untruncated query string would be useful.
>
> I also feel that it would be nice when thinking about situations
> such as troubleshooting a long-running query on production
> environments where we cannot use debuggers.
>
> At that point of the above comment, I was considering exposing
> such information on the shared memory.
> However, since memory contexts are now exposed on the log by
> pg_log_backend_memory_contexts(PID), I'm thinking about
> defining a function that logs the plan of a running query and
> untruncated query string on the specified PID in the same way
> as below.
>
> postgres=# SELECT * FROM pg_log_current_plan(2155192);
> pg_log_current_plan
> ---------------------
> t
> (1 row)
>
> $ tail -f data/log/postgresql-2021-05-12.log
>
> 2021-05-12 17:37:19.481 JST [2155192] LOG: logging the plan of
> running query on PID 2155192
> Query Text: SELECT a.filler FROM pgbench_accounts a JOIN
> pgbench_accounts b ON a.aid = b.aid;
> Merge Join (cost=0.85..83357.85 rows=1000000 width=85)
> Merge Cond: (a.aid = b.aid)
> -> Index Scan using pgbench_accounts_pkey on
> pgbench_accounts a (cost=0.42..42377.43 rows=1000000 width=89)
> -> Index Only Scan using pgbench_accounts_pkey on
> pgbench_accounts b (cost=0.42..25980.42 rows=1000000 width=4)
>
>
> Attached a PoC patch.
>
> Any thoughts?

Great idea. One feature I'd suggest would be adding a 'format' option
as well, if such feature would be feasable.

With regards,

Matthias van de Meent