Re: Status of the table access method work

On Tue, Jun 11, 2019 at 11:47 AM Alvaro Herrera
<alvherre(at)2ndquadrant(dot)com> wrote:
> On 2019-Apr-10, Alexander Korotkov wrote:
> > Alternative idea is to have MVCC-aware indexes. This approach looks
> > more attractive for me. In this approach you basically need xmin,
> > xmax fields in index tuples.
>
> "We liked freezing xmin so much that we had to introduce freezing for
> xmax" -- rhaas dixit. And now we want to introduce freezing for
> indexes?

Plus it would add 8 bytes to the size of every index tuple. if you
are indexing long-ish strings it may not hurt too much, but if your
primary key is an integer, I think your index is going to get a lot
bigger.

The problem with freezing is perhaps avoidable if you store an epoch
in the page special space as part of all this. But I don't see any
way to avoid having the tuples get wider. Possibly you could include
xmin and xmax only when needed, removing xmin once the tuples are
all-visible and splitting pages if you need to make room to add an
xmax. I'm not sure how much that helps, though, because if you do a
bulk insert, you're going to have to leave room for all of the xmins
initially, and removing them later will produce free space for which
you may have little use.

I don't think that including visibility information in indexes is a
bad idea; we've thought about making zheap do this someday. But I
think that we need to use some more sophisticated approach involving,
maybe, undo pointers, or some other kind of magic, rather than just
widening the tuples. I expect that just widening the tuples would be
good enough to win for some use cases, but I think there would be
others that lose heavily.

In any case, I agree that we do not want to create more things that
need freezing.

--
Robert Haas
EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company