Re: Index Skip Scan

At Tue, 24 Sep 2019 17:35:47 +0900 (Tokyo Standard Time), Kyotaro Horiguchi <horikyota(dot)ntt(at)gmail(dot)com> wrote in <20190924(dot)173547(dot)226622711(dot)horikyota(dot)ntt(at)gmail(dot)com>
> At Sun, 22 Sep 2019 23:02:04 -0300, Alvaro Herrera <alvherre(at)2ndquadrant(dot)com> wrote in <20190923020204(dot)GA2781(at)alvherre(dot)pgsql>
> > On 2019-Sep-22, Dmitry Dolgov wrote:
> >
> > > > I think multiplying two ScanDirections to watch for a negative result is
> > > > pretty ugly:
> > >
> > > Probably, but the only alternative I see to check if directions are opposite is
> > > to check that directions come in pairs (back, forth), (forth, back). Is there
> > > an easier way?
> >
> > Maybe use the ^ operator?
>
> It's not a logical operator but a bitwise arithmetic operator,
> which cannot be used if the operands is guaranteed to be 0 or 1
> (in integer). In a-kind-of-standard, but hacky way, "(!a != !b)"
> works as desired since ! is a logical operator.
>
> Wouldn't we use (a && !b) || (!a && b)? Compiler will optimize it
> some good way.

Sorry, it's not a boolean. A tristate value. From the definition
(Back, NoMove, Forward) = (-1, 0, 1), (dir1 == -dir2) if
NoMovement did not exist. If it is not guranteed,

(dir1 != 0 && dir1 == -dir2) ?

regards.

--
Kyotaro Horiguchi
NTT Open Source Software Center