Re: amcheck verification for GiST

From: Andrey Borodin <x4mmm(at)yandex-team(dot)ru>
To: Heikki Linnakangas <hlinnaka(at)iki(dot)fi>
Cc: Peter Geoghegan <pg(at)bowt(dot)ie>, Thomas Munro <thomas(dot)munro(at)enterprisedb(dot)com>, PostgreSQL Hackers <pgsql-hackers(at)postgresql(dot)org>
Subject: Re: amcheck verification for GiST
Date: 2019-03-28 17:08:29
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> 28 марта 2019 г., в 18:35, Andrey Borodin <x4mmm(at)yandex-team(dot)ru> написал(а):
>> Is this really needed? Isn't the ShareLock on the index sufficient? If so, why?
> There may be concurrent inserts? In GiST they can reorder items on page.

Looks like I've tried to cope with same problem twice:
v3 of the patch used AccessShareLock and many locks with incorrect order.
We could use one of possible solutions: either use ShareLock, or rewrite scan to correct locking order.
But patches v4-v7 use both.
I think we should use AccessShareLock, as long as we implemented tricky logic with gist_refind_parent().

>>> + stack->parenttup = gist_refind_parent(rel, stack->parentblk, stack->blkno, strategy);
>> If the gistplacetopage() stuff is truly necessary, then is it okay to
>> call gist_refind_parent() with the original buffer lock still held
>> like this?
> When we call gist_refind_parent() we hold lock for a child and lock parent.
> We exclude concurrent VACUUM, thus parent cannot become a child for current child, because it has to be recycled for such coincidence.
That's merely hard form of paranoia, internal pages are never deleted. gist_index_parent_check() would work just fine with concurrent VACUUM, INSERTs and SELECTs.

Best regards, Andrey Borodin.

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