问题
I try to understand how does overlapping box with infinity works
Why does the following exemple doesn't work. It works fine except when i try to introduce two infinite values.
This exemple should return true.
SELECT box '((1,1),(infinity, 1))' && box '((2, 1),(infinity, 1))' AS overlap;
overlap
---------
f
The two exemples below works
SELECT box '((1,1),(4, 1))' && box '((2, 1),(infinity, 1))' AS overlap;
overlap
---------
t
SELECT box '((1,1),(4, 1))' && box '((2, 1),(5, 1))' AS overlap;
overlap
---------
t
So my question is : Is there something i'm doing wrong or that i don't understand?
回答1:
I asked on the postgresql buglist:
Tom lane gave me this answer :
The test for this involves
FPge(box1->high.x, box2->high.x)
where FPge is defined as
#define FPge(A,B) ((B) - (A) <= EPSILON)
When both high.x values are infinity, you have infinity minus infinity, which yields NaN in IEEE arithmetic, so the comparison to epsilon comes out false.
In general I don't think we promise that geometric operations involving infinite endpoints will behave sanely. There are probably a boatload of corner cases besides this one that'd need to be fixed before we could consider that a supported case.
来源：https://stackoverflow.com/questions/10642646/overlapping-box-with-infinite-edge-coordinate