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From: Heikki Linnakangas <hlinnaka@iki.fi>
To: Andrey Borodin <x4mmm@yandex-team.ru>
Cc: Thomas Munro <thomas.munro@enterprisedb.com>
Cc: pgsql-hackers <pgsql-hackers@postgresql.org>
Cc: Костя Кузнецов <chapaev28@ya.ru>
Subject: Re: GiST VACUUM
Date: Sat, 14 Jul 2018 13:39:56 +0300
Message-ID: <638aa9cf-4e28-a1a4-d2f3-b6697d7736cc@iki.fi> (raw)
In-Reply-To: <B15F859E-D858-4DC4-800C-69B2CB3094BF@yandex-team.ru>
References: <1B9FAC6F-FA19-4A24-8C1B-F4F574844892@yandex-team.ru>
	<CAEepm=1N3qPJqP81U+YwKZYwM53dDU1O-PZ4JmFy03EgGWR+Ng@mail.gmail.com>
	<DFECE345-BEA2-4844-947D-F5875F8E4B06@yandex-team.ru>
	<cd18b07b-f74c-34b2-313d-64ea7dc45f77@iki.fi>
	<21F3E668-4570-4D98-B96D-66ED63A5BCBD@yandex-team.ru>
	<9cc18cc9-08d0-9838-7297-57500311d57b@iki.fi>
	<89E1C149-8E24-4FF6-BF77-6119606AFCA9@yandex-team.ru>
	<1E8C1FFE-D547-427F-BF9E-AD674F5C5DED@yandex-team.ru>
	<50d0105a-bd99-788e-37ee-51f59d81fbec@iki.fi>
	<5F278E38-FD10-45EB-88D0-5B1D7108F984@yandex-team.ru>
	<72ececf1-845d-f6d2-76b8-27d94615f2a8@iki.fi>
	<B15F859E-D858-4DC4-800C-69B2CB3094BF@yandex-team.ru>

On 14/07/18 10:26, Andrey Borodin wrote:
> This is tradeoff between complex concurrency feature and possibility
> of few dead tuples left after VACUUM. I want to understand: is it
> something dangerous in this dead tuples?
Yeah, that's bad. When a new heap tuple is inserted in the same 
location, the old index tuple will point to the new, unrelated, tuple, 
and you will get incorrect query results.

> There is one more serious race condition: result of first scan is 
> just a hint where to look for downlinks to empty pages. If internal 
> page splits between scan and cleanup, offsets of downlinks will be 
> changed, cleanup will lock pages, see non-empty pages and will not 
> delete them (though there are not dead tuples, just not deleted empty
> leafs).

That's fine. Leaving behind a few empty pages is harmless, the next 
vacuum will pick them up.

- Heikki




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  Cc: hlinnaka@iki.fi, x4mmm@yandex-team.ru, thomas.munro@enterprisedb.com, chapaev28@ya.ru
  Subject: Re: GiST VACUUM
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