Re: HOT patch - version 15

Heikki Linnakangas wrote:
> Florian Pflug wrote:
> > Heikki Linnakangas wrote:
> >> Tom Lane wrote:
> >>> Compared to what it currently takes to check the same tuple (a separate
> >>> index entry fetch and traversal to the heap page), this is already an
> >>> enormous performance improvement.
> >>
> >> Though keep in mind that we kill index tuples as soon as they're deemed
> >> to be dead. Nevertheless, I'm not very worried about the cost of
> >> following the chain either. But that's something we can quite easily
> >> measure if we want to.
> >
> > I'm confused now. I though that pruning would be enough to shorten
> > HOT-Chains -
> > because the root line pointer afterwards points directly to the first live
> > tuple. But we can *prune* (without actually defragmenting) without holding
> > a VACUUM-strength lock, right? Or did I get that wrong?
>
> Yes, that's right. You don't seem to be confused at all.
>
> Tom argued that following the tuple chain is cheap enough, and might
> even be cheaper than what we have now, that we don't need to prune just
> for the purpose of keeping the chains short. To which I pointed out that
> currently, without HOT, we mark index tuples pointing to dead tuples as
> killed to avoid following them in the future, so HOT without pruning is
> not cheaper than what we have now.

The central idea I now understand is that pruning only shrinks HOT
chains.  It does not allow reuse of space.  Only defragmentation does
that, and that is triggered when the page is almost full.

--
  Bruce Momjian  <bruce@momjian.us>          http://momjian.us
  EnterpriseDB                               http://www.enterprisedb.com

  + If your life is a hard drive, Christ can be your backup. +