Date: Mon, 17 Aug 2020 19:54:13 -0400
From: Alvaro Herrera <alvherre@2ndquadrant.com>
To: Ibrar Ahmed <ibrar.ahmad@gmail.com>
Cc: Anastasia Lubennikova <a.lubennikova@postgrespro.ru>,
	Robert Haas <robertmhaas@gmail.com>,
	Daniel Gustafsson <daniel@yesql.se>,
	Justin Pryzby <pryzby@telsasoft.com>,
	pgsql-hackers <pgsql-hackers@postgresql.org>,
	Andres Freund <andres@anarazel.de>
Subject: Re: COPY FREEZE and setting PD_ALL_VISIBLE/visibility map bits
Message-ID: <20200817235413.GA11940@alvherre.pgsql>
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On 2020-Aug-14, Ibrar Ahmed wrote:

> The table used for the test contains three columns (integer, text,
> varchar).
> The total number of rows is 10000000 in total.
> 
> Unpatched (Master: 92c12e46d5f1e25fc85608a6d6a19b8f5ea02600)
> COPY: 9069.432 ms vacuum; 2567.961ms
> COPY: 9004.533 ms vacuum: 2553.075ms
> COPY: 8832.422 ms vacuum: 2540.742ms
> 
> Patched (Master: 92c12e46d5f1e25fc85608a6d6a19b8f5ea02600)
> COPY: 10031.723 ms vacuum: 127.524 ms
> COPY: 9985.109  ms vacuum: 39.953 ms
> COPY: 9283.373  ms vacuum: 37.137 ms
> 
> Time to take the copy slightly increased but the vacuum time significantly
> decrease.

"Slightly"?  It seems quite a large performance drop to me -- more than
10%.  Where is that time being spent?  Andres said in [1] that he
thought the performance shouldn't be affected noticeably, but this
doesn't seem to hold true.  As I understand, the idea was that there
would be little or no additional WAL records .. only flags in the
existing record.  So what is happening?

[1] https://postgr.es/m/20190408010427.4l63qr7h2fjcyp77@alap3.anarazel.de

Also, when Andres posted this patch first, he said this was only for
heap_multi_insert because it was a prototype.  But I think we expect
that the table_insert path (CIM_SINGLE mode in copy) should also receive
that treatment.

-- 
Álvaro Herrera                https://www.2ndQuadrant.com/
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services