Re: Slow query after 9.3 to 9.6 migration

On Tue, Dec 27, 2016 at 5:50 PM, Flávio Henrique <yoshimit(at)gmail(dot)com> wrote:
> Hi there, fellow experts!
>
> I need an advice with query that became slower after 9.3 to 9.6 migration.
>
> First of all, I'm from the dev team.
>
> Before migration, we (programmers) made some modifications on query bring
> it's average time from 8s to 2-3s.
>
> As this query is the most executed on our system (it builds the user panel
> to work), every bit that we can squeeze from it will be nice.
>
> Now, after server migration to 9.6 we're experiencing bad times with this
> query again.
>
> Unfortunately, I don't have the old query plain (9.3 version) to show you,
> but in the actual version (9.6) I can see some buffers written that tells me
> that something is wrong.
>
> Our server has 250GB of memory available, but the database team says that
> they can't do nothing to make this query better. I'm not sure, as some
> buffers are written on disk.
>
> Any tip/help will be much appreciated (even from the query side).
>
> Thank you!
>
> The query plan: https://explain.depesz.com/s/5KMn
>
> Note: I tried to add index on kilo_victor table already, but Postgresql
> still thinks that is better to do a seq scan.

Hard to provide more without the query or the 'old' plan. Here are
some things you can try:
*) Set effective_io_concurrency high. You have some heap scanning
going on and this can sometimes help (but it should be marginal).
*) See if you can get any juice out of parallel query
*) try playing with enable_nestloop and enable_seqscan. these are
hail mary passes but worth a shot.

Run the query back to back with same arguments in the same database
session. Does performance improve?

Big gains (if any) are likely due to indexing strategy.
I do see some suspicious casting, for example:

Join Filter: ((four_charlie.delta_tango)::integer =
(six_quebec.golf_bravo)::integer)

Are you casting in the query or joining through dissimilar data types?
I suspect your database team might be incorrect.

merlin