Re: Index range search optimization

On Fri, 22 Sept 2023 at 00:48, Peter Geoghegan <pg(at)bowt(dot)ie> wrote:
>
> On Thu, Sep 21, 2023 at 5:11 AM Pavel Borisov <pashkin(dot)elfe(at)gmail(dot)com> wrote:
> > I looked at the patch code and I agree with this optimization.
> > Implementation also looks good to me except change :
> > + if (key->sk_flags & (SK_BT_REQFWD | SK_BT_REQBKWD) &&
> > + !(key->sk_flags & SK_ROW_HEADER))
> > + requiredDir = true;
> > ...
> > - if ((key->sk_flags & SK_BT_REQFWD) &&
> > - ScanDirectionIsForward(dir))
> > - *continuescan = false;
> > - else if ((key->sk_flags & SK_BT_REQBKWD) &&
> > - ScanDirectionIsBackward(dir))
> > + if (requiredDir)
> > *continuescan = false;
> >
> > looks like changing behavior in the case when key->sk_flags &
> > SK_BT_REQFWD && (! ScanDirectionIsForward(dir)) &&
> > (!requiredDirMatched)
> > Originally it doesn't set *continuescan = false; and with the patch it will set.
>
> I agree that this is a problem. Inequality strategy scan keys are used
> when the initial positioning strategy used by _bt_first (for its
> _bt_search call) is based on an operator other than the "=" operator
> for the opclass. These scan keys are required in one direction only
> (Konstantin's original patch just focussed on these cases, actually).
> Obviously, that difference matters. I don't think that this patch
> should do anything that even looks like it might be revising the
> formal definition of "required in the current scan direction".
I think it's the simplification that changed code behavior - just an
overlook and this could be fixed easily.

> Also, requiredDirMatched isn't initialized by _bt_readpage() when
> "so->firstPage". Shouldn't it be initialized to false?
True.

> > Also naming of requiredDirMatched and requiredDir seems semantically
> > hard to understand the meaning without looking at the patch commit
> > message. But I don't have better proposals yet, so maybe it's
> > acceptable.
>
> I agree. How about "requiredMatchedByPrecheck" instead of
> "requiredDirMatched", and "required" instead of "requiredDir"?
For me, the main semantic meaning is omitted and even more unclear,
i.e. what exactly required and matched. I'd suppose scanDirRequired,
scanDirMatched, but feel it's not ideal either. Or maybe trySkipRange,
canSkipRange etc.

Regards,
Pavel Borisov,
Supabase.