Re: Making all nbtree entries unique by having heap TIDs participate in comparisons

On Mon, Jun 18, 2018 at 7:57 AM, Claudio Freire <klaussfreire(at)gmail(dot)com> wrote:
> Way back when I was dabbling in this kind of endeavor, my main idea to
> counteract that, and possibly improve performance overall, was a
> microvacuum kind of thing that would do some on-demand cleanup to
> remove duplicates or make room before page splits. Since nbtree
> uniqueification enables efficient retail deletions, that could end up
> as a net win.

That sounds like a mechanism that works a bit like
_bt_vacuum_one_page(), which we run at the last second before a page
split. We do this to see if a page split that looks necessary can
actually be avoided.

I imagine that retail index tuple deletion (the whole point of this
project) would be run by a VACUUM-like process that kills tuples that
are dead to everyone. Even with something like zheap, you cannot just
delete index tuples until you establish that they're truly dead. I
guess that the delete marking stuff that Robert mentioned marks tuples
as dead when the deleting transaction commits. Maybe we could justify
having _bt_vacuum_one_page() do cleanup to those tuples (i.e. check if
they're visible to anyone, and if not recycle), because we at least
know that the deleting transaction committed there. That is, they
could be recently dead or dead, and it may be worth going to the extra
trouble of checking which when we know that it's one of the two
possibilities.

--
Peter Geoghegan