Horton

Re: GiST VACUUM

Heikki Linnakangas <hlinnaka@iki.fi>

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From: Heikki Linnakangas <hlinnaka@iki.fi>
To: Andrey Borodin <x4mmm@yandex-team.ru>
Cc: Thomas Munro <thomas.munro@enterprisedb.com>, pgsql-hackers <pgsql-hackers@postgresql.org>, Костя Кузнецов <chapaev28@ya.ru>
Date: 2018-07-14T10:39:56Z
Lists: pgsql-hackers 
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

Commits

  1. Use full 64-bit XID for checking if a deleted GiST page is old enough.

  2. Refactor checks for deleted GiST pages.

  3. Delete empty pages during GiST VACUUM.

  4. Scan GiST indexes in physical order during VACUUM.

  5. Prevent GIN deleted pages from being reclaimed too early