[postgis-tickets] [PostGIS] #5361: Compound Curve inconsistent behavior with ST_GeometryN, ST_NumGeometries, and ST_Dump

[PostGIS]

#5361: Compound Curve inconsistent behavior with ST_GeometryN, ST_NumGeometries,
and ST_Dump
---------------------+---------------------------
 Reporter:  robe     |      Owner:  pramsey
     Type:  defect   |     Status:  new
 Priority:  low      |  Milestone:  PostGIS 3.4.0
Component:  postgis  |    Version:  master
 Keywords:           |
---------------------+---------------------------
 Someone asked a question on IRC, which caused me some concern.

 How do you get the child types of a compound curve?

 I was bothered because of the dichotomy of functions that should agree
 with each other that don't

 {{{
 SELECT ST_Numgeometries(geom), ST_GeometryType(ST_GeometryN(geom,1)),
 ST_GeometryN(geom,2)
 FROM ST_GeomFromText(
 'COMPOUNDCURVE(
 (2 2, 2.5 2.5),
 CIRCULARSTRING(2.5 2.5, 4.5 2.5, 3.5 3.5),
 (3.5 3.5, 2.5 4.5, 3 5)
 )') AS f(geom);
 }}}


 Output is:


 {{{
 st_numgeometries | st_geometrytype  | st_geometryn
 ------------------+------------------+--------------
                 3 | ST_CompoundCurve |
 (1 row)
 }}}


 One can argue both sides, that a compoundcurve is no different from a
 LINESTRING in that it defines a contiguous line of sorts.

 But yet both ST_Numgeometries and ST_Dump do not see it that way

 {{{
 SELECT gd.path,  ST_GeometryType(gd.geom)
 FROM ST_GeomFromText(
 'COMPOUNDCURVE(
 (2 2, 2.5 2.5),
 CIRCULARSTRING(2.5 2.5, 4.5 2.5, 3.5 3.5),
 (3.5 3.5, 2.5 4.5, 3 5)
 )') AS f(geom), ST_Dump(f.geom) AS gd;
 }}}



 {{{
 path |  st_geometrytype
 ------+-------------------
  {1}  | ST_LineString
  {2}  | ST_CircularString
  {3}  | ST_LineString
 (3 rows)
 }}}



 But then if ST_Dump were not flawed in this one, one could not arrive at
 the subtypes of a compound curve.  What of ST_NumGeometries?  Do the specs
 say anything about how compound curve sub-elements should be counted.
-- 
Ticket URL: <https://trac.osgeo.org/postgis/ticket/5361>
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