Re: [HACKERS] [WIP] Effective storage of duplicates in B-tree index.

From: Heikki Linnakangas <hlinnaka(at)iki(dot)fi>
To: Peter Geoghegan <pg(at)bowt(dot)ie>, Anastasia Lubennikova <a(dot)lubennikova(at)postgrespro(dot)ru>
Cc: PostgreSQL Hackers <pgsql-hackers(at)lists(dot)postgresql(dot)org>
Subject: Re: [HACKERS] [WIP] Effective storage of duplicates in B-tree index.
Date: 2020-01-08 13:56:23
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On 04/01/2020 03:47, Peter Geoghegan wrote:
> Attached is v28, which fixes bitrot from my recent commits to refactor
> VACUUM-related code in nbtpage.c.

I started to read through this gigantic patch. I got about 1/3 way
through. I wrote minor comments directly in the attached patch file,
search for "HEIKKI:". I wrote them as I read the patch from beginning to
end, so it's possible that some of my questions are answered later in
the patch. I didn't have the stamina to read through the whole patch
yet, I'll continue later.

One major design question here is about the LP_DEAD tuples. There's
quite a lot of logic and heuristics and explanations related to unique
indexes. To make them behave differently from non-unique indexes, to
keep the LP_DEAD optimization effective. What if we had a separate
LP_DEAD flag for every item in a posting list, instead? I think we
wouldn't need to treat unique indexes differently from non-unique
indexes, then. I tried to search this thread to see if that had been
discussed already, but I didn't see anyone proposing that approach.

Another important decision here is the on-disk format of these tuples.
The format of IndexTuples on a b-tree page has become really
complicated. The v12 changes to store TIDs in order did a lot of that,
but this makes it even more complicated. I know there are strong
backwards-compatibility reasons for the current format, but
nevertheless, if we were to design this from scratch, what would the
B-tree page and tuple format be like?

- Heikki

Attachment Content-Type Size
v28-0001-Add-deduplication-to-nbtree.patch-with-heikki-comments text/plain 211.6 KB

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