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From: Tom Lane <tgl@sss.pgh.pa.us>
To: Laurenz Albe <laurenz.albe@cybertec.at>
Cc: Jeff Janes <jeff.janes@gmail.com>
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 08:44:14 -0500
Message-ID: <28392.1576676654@sss.pgh.pa.us> (raw)
In-Reply-To: <2c41458116a7b80fa8eba117d14fdbd5489c4847.camel@cybertec.at>
References: <CABSfb-6iUvpGCzcgni+tVoiUWTc9aQdUtKbOzFzGqJcypiLa4g@mail.gmail.com>
	<adb87d39c5faa26d85274824ea4dd15ffc44ac40.camel@cybertec.at>
	<CABSfb-55VAcUwvy+PtsBsEk8EMfODtC6p65qVHSYhOqWWf80mA@mail.gmail.com>
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	<e7acf6bcdf5f267d8195377e74bd12005ce5949d.camel@cybertec.at>
	<CAMkU=1w2i=Ph7840GYtf2eWta858Y3Eyb0zRKJd84cyEEctBZg@mail.gmail.com>
	<2c41458116a7b80fa8eba117d14fdbd5489c4847.camel@cybertec.at>

Laurenz Albe <laurenz.albe@cybertec.at> writes:
> On Tue, 2019-12-17 at 11:11 -0500, Jeff Janes wrote:
>> 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.

Nonetheless, the presented test case consists of repeatedly doing
the same query, in a fresh session each time.  If there's not other
activity then this should reach some sort of steady state.  The
table is apparently fairly large, so I don't find it surprising
that the steady state fails to be 100% cached.

			regards, tom lane





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