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From: Justin Pryzby <pryzby@telsasoft.com>
To: Vladimir Ryabtsev <greatvovan@gmail.com>
Cc: f.pardi@portavita.eu
Cc: pgsql-performance@lists.postgresql.org
Subject: Re: Why could different data in a table be processed with different performance?
Date: Mon, 24 Sep 2018 21:19:54 -0500
Message-ID: <20180925021954.GW2471@telsasoft.com> (raw)
In-Reply-To: <CAMqTPq=f2fQ8tnUu3UxrvfogiDCn+04HWFX8fpZkDoeYQ7ktTQ@mail.gmail.com>
References: <CAMqTPq=aFwfiqv=P9EDs+UxeOgfe03kyQ0uvtUHvSxvfSvYR+w@mail.gmail.com>
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	<e8d93055-9643-0ad2-d8bd-3f4fc344bf51@portavita.eu>
	<CAMqTPqnUB54YXQutPKPWdoUKEZOnV58WndfTvZagrNLiTr6RHw@mail.gmail.com>
	<20180924233423.GV2471@telsasoft.com>
	<CAMqTPq=f2fQ8tnUu3UxrvfogiDCn+04HWFX8fpZkDoeYQ7ktTQ@mail.gmail.com>

On Mon, Sep 24, 2018 at 05:59:12PM -0700, Vladimir Ryabtsev wrote:
> > I *suspect* VACUUM FULL won't help, since (AIUI) it copies all "visible"
...
> I am going copy the slow range into a table nearby and see if it reproduces
> (I hope "INSERT INTO t2 SELECT * FROM t1 WHERE ..." will keep existing
> order of rows). Then I could try the same after CLUSTER.

If it does an index scan, I think that will badly fail to keep the same order
of heap TIDs - it'll be inserting rows in ID order rather than in (I guess)
reverse ID order.

Justin




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