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From: Laurenz Albe <laurenz.albe@cybertec.at>
To: Jeff Janes <jeff.janes@gmail.com>
Cc: Tom Lane <tgl@sss.pgh.pa.us>
Cc: Peter Geoghegan <pg@bowt.ie>
Cc: Shijia Wei <shijiawei@utexas.edu>
Cc: Pgsql Performance <pgsql-performance@lists.postgresql.org>
Subject: Re: Consecutive Query Executions with Increasing Execution Time
Date: Wed, 18 Dec 2019 13:17:10 +0100
Message-ID: <2c41458116a7b80fa8eba117d14fdbd5489c4847.camel@cybertec.at> (raw)
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	<19575.1576529417@sss.pgh.pa.us>
	<e7acf6bcdf5f267d8195377e74bd12005ce5949d.camel@cybertec.at>
	<CAMkU=1w2i=Ph7840GYtf2eWta858Y3Eyb0zRKJd84cyEEctBZg@mail.gmail.com>

On Tue, 2019-12-17 at 11:11 -0500, Jeff Janes wrote:
> On Tue, Dec 17, 2019 at 8:08 AM Laurenz Albe <laurenz.albe@cybertec.at> wrote:
> > On Mon, 2019-12-16 at 15:50 -0500, Tom Lane wrote:
> > > Peter Geoghegan <pg@bowt.ie> writes:
> > > > Why do the first and the twentieth executions of the query have almost
> > > > identical "buffers shared/read" numbers? That seems odd.
> > > 
> > > It's repeat execution of the same query, so that doesn't seem odd to me.
> > 
> > Really?  Shouldn't the blocks be in shared buffers after a couple
> > of executions?
> 
> If it is doing a seq scan (I don't know if it is) they intentionally use a
> small ring buffer to, so they evict their own recently used blocks, rather
> than evicting other people's blocks.  So these blocks won't build up in
> shared_buffers very rapidly just on the basis of repeated seq scans.

Sure, but according to the execution plans it is doing a Parallel Index Only Scan.

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






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