Re: Index Skip Scan

Thank you very much for the benchmarking!

A bit different topic from the latest branch..

At Sat, 8 Feb 2020 14:11:59 +0100, Tomas Vondra <tomas(dot)vondra(at)2ndquadrant(dot)com> wrote in
> >Yes, I've mentioned that already in one of the previous emails :) The
> >simplest way I see to achieve what we want is to do something like in
> >attached modified version with a new hasDeclaredCursor field. It's not
> >a
> >final version though, but posted just for discussion, so feel free to
> >suggest any improvements or alternatives.
>
> IMO the proper fix for this case (moving forward, reading backwards)
> is
> simply making it work by properly checking deleted tuples etc. Not
> sure
> why that would be so much complex (haven't tried implementing it)?

I don't think it's not so complex. But I suspect that it might be a
bit harder starting from the current shpae.

The first attached (renamed to .txt not to confuse the cfbots) is a
small patch that makes sure if _bt_readpage is called with the proper
condition as written in its comment, that is, caller must have pinned
and read-locked so->currPos.buf. This patch reveals many instances of
breakage of the contract.

The second is a crude fix the breakages, but the result seems far from
neat.. I think we need rethinking taking modification of support
functions into consideration.

> I think making this depend on things like declared cursor etc. is
> going
> to be tricky, may easily be more complex than checking deleted tuples,
> and the behavior may be quite surprising.

Sure.

regards.

--
Kyotaro Horiguchi
NTT Open Source Software Center