Re: GiST VACUUM

On 13/07/18 21:28, Andrey Borodin wrote:
>> 13 июля 2018 г., в 18:25, Heikki Linnakangas <hlinnaka@iki.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