Message-ID: <1515574307.2848.20.camel@cybertec.at>
Subject: Re: Performance of a Query
From: Laurenz Albe <laurenz.albe@cybertec.at>
To: Scott Marlowe <scott.marlowe@gmail.com>, "Kumar, Virendra"
	 <Virendra.Kumar@guycarp.com>
Cc: "pgsql-performance@postgresql.org" <pgsql-performance@postgresql.org>
Date: Wed, 10 Jan 2018 09:51:47 +0100
In-Reply-To: 
 <CAOR=d=0cPhUjHhX4wRtBGX6Fu3NRKfMDi4YdTs1nh6DVKeE+Xw@mail.gmail.com>
References: <7b438042521b45f89e510c20a798d22b@USFKL11XG20CN01.mercer.com>
	 <CAOR=d=0cPhUjHhX4wRtBGX6Fu3NRKfMDi4YdTs1nh6DVKeE+Xw@mail.gmail.com>
Organization: Cybertec
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Scott Marlowe wrote:
> On Tue, Jan 9, 2018 at 2:18 PM, Kumar, Virendra
> <Virendra.Kumar@guycarp.com> wrote:
> > Hello Gurus,
> > 
> > I am struggling to tune a query which is doing join on top of aggregate for
> > around 3 million rows. The plan and SQL is attached to the email.
> > 
> > Below is system Details:
> > 
> > PGSQL version – 10.1
> > 
> > OS – RHEL 3.10.0-693.5.2.el7.x86_64
> > 
> > Binary – Dowloaded from postgres.org compiled and installed.
> > 
> > Hardware – Virtual Machine with 8vCPU and 32GB of RAM, on XFS filesystem.
> 
> I uploaded your query plan here: https://explain.depesz.com/s/14r6
> 
> The most expensive part is the merge join at the end.
> 
> Lines like this one: "Buffers: shared hit=676 read=306596, temp
> read=135840 written=135972"
> 
> Tell me that your sorts etc are spilling to disk, so the first thing
> to try is upping work_mem a bit. Don't go crazy, as it can run your
> machine out of memory if you do. but doubling or tripling it and
> seeing the effect on the query performance is a good place to start.
> 
> The good news is that most of your row estimates are about right, so
> the query planner is doing what it can to make the query fast, but I'm
> guessing if you get the work_mem high enough it will switch from a
> merge join to a hash_join or something more efficient for large
> numbers of rows.

Looking at the plan, I'd guess that the following index could be helpful:

CREATE INDEX ON ap.site_exposure(portfolio_id, peril_id, account_id);

Don't know how much it would buy you, but you could avoid the
sequential scan and the sort that way.

Yours,
Laurenz Albe