[PostGIS] #5838: Possible bug in ST_Intersects

[PostGIS]

#5838: Possible bug in ST_Intersects
---------------------+---------------------------
 Reporter:  rouen    |      Owner:  pramsey
     Type:  defect   |     Status:  new
 Priority:  medium   |  Milestone:  PostGIS 3.5.2
Component:  postgis  |    Version:  3.5.x
 Keywords:           |
---------------------+---------------------------
 {{{POSTGIS="3.5.1 48ab069" [EXTENSION] PGSQL="160" GEOS="3.8.0-CAPI-1.13.1
 " PROJ="6.3.1" LIBXML="2.9.10" LIBJSON="0.13.1" LIBPROTOBUF="1.3.3"
 WAGYU="0.5.0 (Internal)"}}}

 Let's have 2 lines in chain (endpoint of one is startpoint of the other)
 and 3rd line intersecting (one of) them very close to that point.
 ST_Intersects behaves badly in this case:
 {{{
 WITH data AS
 (
   SELECT 'a' AS id, st_geomfromtext('LINESTRING(20.3580062
 48.3096067,20.3579669 48.3096863)',4326) AS geom
   UNION
   SELECT 'b' AS id, st_geomfromtext('LINESTRING(20.3579669
 48.3096863,20.3579264 48.3097684)',4326) AS geom
   UNION
   SELECT 'c' AS id, st_geomfromtext('LINESTRING(20.3580084
 48.3097278,20.3579583 48.3096777)',4326) AS geom
 )
 SELECT
   st_intersects(a.geom,b.geom) AS ab_intersects, -- true
   st_intersects(a.geom,c.geom) AS ac_intersects, -- false
   st_intersects(b.geom,c.geom) AS bc_intersects -- false
 FROM data a, data b, data c
 WHERE a.id = 'a' and b.id = 'b' and c.id = 'c';
 }}}

 This alone is wrong (c is clearly crossing the chain of a+b).
 But interestingly enough, with JOIN it gives different results (c does
 intersects a):

 {{{
 WITH data AS
 (
   SELECT 'a' AS id, st_geomfromtext('LINESTRING(20.3580062
 48.3096067,20.3579669 48.3096863)',4326) AS geom
   UNION
   SELECT 'b' AS id, st_geomfromtext('LINESTRING(20.3579669
 48.3096863,20.3579264 48.3097684)',4326) AS geom
   UNION
   SELECT 'c' AS id, st_geomfromtext('LINESTRING(20.3580084
 48.3097278,20.3579583 48.3096777)',4326) AS geom
 )
 SELECT c.id, x.id -- 3 + 1
 FROM data c
 JOIN data x ON st_intersects(c.geom,x.geom)
 WHERE c.id='c' and x.id<>'c'
 }}}
-- 
Ticket URL: <https://trac.osgeo.org/postgis/ticket/5838>
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