ST_DFullyWithin

[Paul Ramsey]

In fixing https://trac.osgeo.org/postgis/ticket/5639, it has become clear to me that our definition of ST_DFullyWithin is probably “wrong”, that is, it’s not what people think they are getting when they call it. 

Right now, ST_DFullyWithin(A, B, radius) can be expressed as: 
  ST_MaxDistance(A, B) < radius

However, the distance https://postgis.net/docs/ST_MaxDistance.html returns is the maximum distance between the vertices of A and the vertices of B. In the limit, for example, the ST_MaxDistance(A, A) is not zero, it’s the longest distance between any two vertices of A. This is fine, there’s a place for this number, but applied to ST_DFullyWithin, it doesn’t return what I think people expect.

Another way of expressing ST_DWithin(A, B, radius) is: 
  ST_Intersects(ST_Buffer(A, radius), B)

Another way of expressing what I *think* people expect from ST_DFullyWithin(A, B, radius) is: 
  ST_Contains(ST_Buffer(A), B)

The canonical use case of something like ST_DFullyWithin(A, B, radius) would be checking to see if two line segments were “similar”, so a road conflation problem.  The current implementation will very much not help with that.

I propose changing ST_DFullyWithin to 
(a) apply the buffer recipe
(b) once a new version of GEOS with an oriented hausdorf is available, use that implementation instead to calculate the distance metric the buffer recipe implies

The function will remain index-enabled, fortunately that logic still makes sense.

P
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