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

On Mon, Jun 18, 2018 at 10:33 PM, Peter Geoghegan <pg(at)bowt(dot)ie> wrote:
> 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.
>

No, I don't think that is the case because we want to perform in-place
updates for indexed-column-updates. If we won't delete-mark the index
tuple before performing in-place update, then we will have two tuples
in the index which point to the same heap-TID.

--
With Regards,
Amit Kapila.
EnterpriseDB: http://www.enterprisedb.com