Re: Slow query after 9.3 to 9.6 migration

On Tue, Dec 27, 2016 at 3:50 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.

The second sorts etc start spilling to disk your performance is gonna
tank. Try increasing work_mem to something moderate like 256M to 1G.
Note that work_mem is per sort / action, so if you got 100 users
running queries with 2 or 3 sorts at a time you can exhaust memory
real fast. OTOH, a db with proper pooling on connections etc (aka 10
to 20 live connections at a time) cna easily handle 1G work_mem if
it's got 256G RAM