[postgis-tickets] [PostGIS] #3949: Infinite (?) loop in distance calculation

[PostGIS]

#3949: Infinite (?) loop in distance calculation
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  Reporter:  pramsey  |      Owner:  nicklas
      Type:  defect   |     Status:  assigned
  Priority:  high     |  Milestone:  PostGIS 2.4.3
 Component:  postgis  |    Version:  trunk
Resolution:           |   Keywords:
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Comment (by nicklas):

 Just to further illustrate the characteristics of the algorithm

 If you move the geometries further apart it will be faster because of the
 angels between different vertex-points. It makes it easier to order the
 points in a straight line:

 {{{
 drop table if exists t; create table t as select gid, geom from
 inf_distance;
 update t set geom = ST_Translate(geom, -20, -20) where gid = 7;

 select st_distance(a.geom, b.geom) from
 (select * from t where gid = 7) a,
 (select * from t where gid = 37) b
 3 sec
 }}}

 and when facing the opposite side of the geometries towards each other
 instead it starts to go really fast (brute force would still use 1 min 19
 sec on this)

 {{{
 drop table if exists t; create table t as select gid, geom from
 inf_distance;
 update t set geom = ST_Translate(geom, -20, -20) where gid = 37;

 select st_distance(a.geom, b.geom) from
 (select * from t where gid = 7) a,
 (select * from t where gid = 37) b
 32 ms
 }}}

 This makes todays distance function very unpredictable from a user
 perspective.
 But that is limitations of the algorithm and I guess it anyway is
 preferable over brute force since it gives such a great boost in cases
 where it fits. About distance between the geometries it is the relation
 between the geometries related to the "width" of the geometries from the
 other geometries perspective that matters. So small geometries (not few
 points but small) requires a shorter distance to work well.

 This makes sense if looking at the wiki-post about the method
 https://trac.osgeo.org/postgis/wiki/NewDistCalcGeom2Geom

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Ticket URL: <https://trac.osgeo.org/postgis/ticket/3949#comment:7>
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