Re: Deleting older versions in unique indexes to avoid page splits

On Mon, Jan 18, 2021 at 1:10 PM Victor Yegorov <vyegorov@gmail.com> wrote:
> I must admit, that it's a bit difficult to understand you here (at least for me).
>
> I assume that by "bet" you mean flagged tuple, that we marked as such
> (should we implement the suggested case).
> As heapam will give up early in case there are no deletable tuples, why do say,
> that "bet" is no longer low? At the end, we still decide between page split (and
> index bloat) vs a beneficial space cleanup.

Well, as I said, there are various ways in which our inferences (say
the ones in nbtdedup.c) are likely to be wrong. You understand this
already. For example, obviously if there are two duplicate index
tuples pointing to the same heap page then it's unlikely that both
will be deletable, and there is even a fair chance that neither will
be (for many reasons). I think that it's important to justify why we
use stuff like that to drive our decisions -- the reasoning really
matters. It's very much not like the usual optimization problem thing.
It's a tricky thing to discuss.

I don't assume that I understand all workloads, or how I might
introduce regressions. It follows that I should be extremely
conservative about imposing new costs here. It's good that we
currently know of no workloads that the patch is likely to regress,
but absence of evidence isn't evidence of absence. I personally will
never vote for a theoretical risk with only a theoretical benefit. And
right now that's what the idea of doing bottom-up deletions in more
marginal cases (the page flag thing) looks like.

--
Peter Geoghegan