Re: Hash Indexes

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От Jeff Janes
Тема Re: Hash Indexes
Дата
Msg-id CAMkU=1xu6nyVLhgiDRX342oG-a+e-rXYFuf9y-8M13+QuSnVOA@mail.gmail.com
обсуждение исходный текст
Ответ на Re: Hash Indexes  (Jeff Janes <jeff.janes@gmail.com>)
Ответы Re: Hash Indexes  (Amit Kapila <amit.kapila16@gmail.com>)
Список pgsql-hackers
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On Tue, Sep 13, 2016 at 9:31 AM, Jeff Janes <jeff.janes@gmail.com> wrote:




=======

+Vacuum acquires cleanup lock on bucket to remove the dead tuples and or tuples
+that are moved due to split.  The need for cleanup lock to remove dead tuples
+is to ensure that scans' returns correct results.  Scan that returns multiple
+tuples from the same bucket page always restart the scan from the previous
+offset number from which it has returned last tuple.

Perhaps it would be better to teach scans to restart anywhere on the page, than to force more cleanup locks to be taken?

Commenting on one of my own questions: 

This won't work when the vacuum removes the tuple which an existing scan is currently examining and thus will be used to re-find it's position when it realizes it is not visible and so takes up the scan again.

The index tuples in a page are stored sorted just by hash value, not by the combination of (hash value, tid).  If they were sorted by both, we could re-find our position even if the tuple had been removed, because we would know to start at the slot adjacent to where the missing tuple would be were it not removed. But unless we are willing to break pg_upgrade, there is no feasible way to change that now.

Cheers,

Jeff

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