Re: [HACKERS] compress method for spgist - 2 - Mailing list pgsql-hackers

On 20.09.2017 23:19, Alexander Korotkov wrote:

On Wed, Sep 20, 2017 at 11:07 PM, Tom Lane <tgl@sss.pgh.pa.us> wrote:

Darafei Praliaskouski <me@komzpa.net> writes:
> I have some questions about the circles example though.

>  * What is the reason for isnan check and swap of box ordinates for circle? It wasn't in the code previously.

I hadn't paid any attention to this patch previously, but this comment
excited my curiosity, so I went and looked:

+       bbox->high.x = circle->center.x + circle->radius;
+       bbox->low.x = circle->center.x - circle->radius;
+       bbox->high.y = circle->center.y + circle->radius;
+       bbox->low.y = circle->center.y - circle->radius;
+
+       if (isnan(bbox->low.x))
+       {
+               double tmp = bbox->low.x;
+               bbox->low.x = bbox->high.x;
+               bbox->high.x = tmp;
+       }

Maybe I'm missing something, but it appears to me that it's impossible for
bbox->low.x to be NaN unless circle->center.x and/or circle->radius is a
NaN, in which case bbox->high.x would also have been computed as a NaN,
making the swap entirely useless.  Likewise for the Y case.  There may be
something useful to do about NaNs here, but this doesn't seem like it.

 

Yeah, +1.

It is possible for bbox->low.x to be NaN when circle->center.x is and
circle->radius are both +Infinity.  Without this float-order-preserving swapping
one regression test for KNN with ORDER BY index will be totally broken (you can
try it: https://github.com/glukhovn/postgres/tree/knn).  Unfortunately, I do not
remember exactly why, but most likely because of the incorrect index structure.

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