From: Heikki Linnakangas <hlinnaka(at)iki(dot)fi>
To: Andrey Borodin <x4mmm(at)yandex-team(dot)ru>
Cc: Thomas Munro <thomas(dot)munro(at)enterprisedb(dot)com>, pgsql-hackers <pgsql-hackers(at)postgresql(dot)org>, Костя Кузнецов <chapaev28(at)ya(dot)ru>
Subject: Re: GiST VACUUM
Date: 2018-07-13 20:28:41
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On 13/07/18 21:28, Andrey Borodin wrote:
>> 13 июля 2018 г., в 18:25, Heikki Linnakangas <hlinnaka(at)iki(dot)fi>
>> написал(а):
>> Looking at the second patch, to scan the GiST index in physical
>> order, that seems totally unsafe, if there are any concurrent page
>> splits. In the logical scan, pushStackIfSplited() deals with that,
>> by comparing the page's NSN with the parent's LSN. But I don't see
>> anything like that in the physical scan code.
> Leaf page can be pointed by internal page and rightlink
> simultaneously. The purpose of NSN is to visit this page exactly once
> by following only on of two links in a scan. This is achieved
> naturally if we read everything from the beginning to the end. (That
> is how I understand, I can be wrong)

The scenario where this fails goes like this:

1. Vacuum scans physical pages 1-10
2. A concurrent insertion splits page 15. The new left half stays on
page 15, but the new right half goes to page 5
3. Vacuum scans pages 11-20

Now, if there were any dead tuples on the right half of the split, moved
to page 5, the vacuum would miss them.

The way this is handled in B-tree is that when a page is split, the page
is stamped with current "vacuum cycle id". When the vacuum scan
encounters a page with the current cycle id, whose right-link points to
a lower-numbered page, it immediately follows the right link, and
re-scans it. I.e. in the above example, if it was a B-tree, in step 3
when vacuum scans page 15, it would see that it was concurrently split.
It would immediately vacuum page 5 again, before continuing the scan in
physical order.

We'll need to do something similar in GiST.

- Heikki

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