Re: Clarification about HOT

On 11/2/07, Martijn van Oosterhout <kleptog(at)svana(dot)org> wrote:
>
> On Fri, Nov 02, 2007 at 06:12:37PM +0530, Gokulakannan Somasundaram wrote:
> > I am especially interested in the case of continuing the HOT chain
> across
> > pages. When we are actually reclaiming space, we should check the
> snapshot
> > and reclaim it. If it is HOT updated, we will leave the top most tuple
> and
> > take the rest. So then the top most tuple will remain always and any
> index
> > scan now has to make read two heap pages to reach the target entry.Isthis
> > the only reason, it was left out?
>
> It's not that simple. At any point in time there may be dozens of
> active snapshots, each of which might see a different tuple in the
> chain. So to clear any tuple you have to wait until all active
> snapshots are gone. You will almost never be able to reduce the chain
> to just one.

I understand that if you have to Vacuum a tuple, it has to satisfy the
necessary snapshot requirements. i will never be able to reduce the chain to
just one, because the there is always a indirection at the top of HOT. I
understood this.

My question was is it the only reason for the decision to stop HOT across
pages.

As for your original question and jumping across pages, why stop at
> one. Why not chain HOT tuples down 100 pages? Because then it gets very
> expensive. Not to mention the locking considerations. Better keep it
> simple.

If you have to jump 100 pages, then you have 100 versions of the same tuple,
which are not to be Vacuumed. That's a heavily updated tuple indeed. Then
you will have 100 index tuples and you will anyway visit all those versions
in a normal index scan. The question is whether you want to visit it through
the HOT chain/ through the index entries. If you visit it through HOT chain,
indexes can be in reduced size.

--
Thanks,
Gokul.
CertoSQL Project,
Allied Solution Group.
(www.alliedgroups.com)