On 11/2/07, Martijn van Oosterhout <kleptog@svana.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.Is this
> 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)