Re: Heap WARM Tuples - Design Draft

On 2016-08-04 18:11:17 +0100, Simon Riggs wrote:
> On 4 August 2016 at 17:31, Andres Freund <andres@anarazel.de> wrote:
> > Hi,
> >
> > On 2016-08-04 16:29:09 +0530, Pavan Deolasee wrote:
> >> Indexes whose values do not change do not require new index pointers. Only
> >> the index whose key is being changed will need a new index entry. The new
> >> index entry will be set to the CTID of the root line pointer.
> >
> > That seems to require tracing all hot-chains in a page, to actually
> > figure out what the root line pointer of a warm-updated HOT tuple is,
> > provided it's HOT_UPDATED itself.  Or have you found a smart way to
> > figure that out?
> 
> Hmm, sorry, I did think of that point and I thought I had added it to the doc.
> 
> So, yes, I agree - re-locating the root is the biggest downside to
> this idea. Perhaps Pavan has other thoughts?
> 
> I think its doable, but it will be fiddly.

I'm less afraid of the fiddlyness of finding the root tuple, than the
cost. It's not cheap to walk through, potentially, all hot chains to
find the root ctid.

Have you considered what it'd take to allow index pointers to allow to
point to "intermediate" root tuples? I.e. have some indexes point into
the middle of an update-chain, whereas others still point to the
beginning? There's certainly some complications involved with that, but
it'd also have the advantage in reducing the amount of rechecking that
needs to be done.