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

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