[postgis-tickets] [PostGIS] #4181: St_AsMVTGeom: Avoid type change due to validation

[PostGIS]

#4181: St_AsMVTGeom: Avoid type change due to validation
-------------------------+---------------------------
  Reporter:  Algunenano  |      Owner:  Algunenano
      Type:  defect      |     Status:  reopened
  Priority:  medium      |  Milestone:  PostGIS 2.4.6
 Component:  postgis     |    Version:  2.4.x
Resolution:              |   Keywords:
-------------------------+---------------------------

Comment (by Algunenano):

 The steps taken for that geometry:
 * INPUT: A valid multipolygon made of 2 polygons:
 `SRID=3857;MULTIPOLYGON(((-8230324.85568 4984496.35686,-8230307.11142
 4984654.46474,-8230285.21086 4984959.6035,-8230324.85568
 4984496.35686)),((-8230327.54014 4984444.33052,-8230327.23971
 4984450.39402,-8230327.26833 4984449.87732,-8230327.54014
 4984444.33052)))`

 * After the simplification step it stays the same (it was already
 simplified).
 * Clipping wasn't necessary, so it also stays the same.

 * After snapping it to the MVT grid we have 2 polygons, with the second
 one being invalid:
 `SRID=3857;MULTIPOLYGON(((3245 2224,3253 2158,3262 2030,3245 2224)),((3244
 2246,3244 2243,3244 2244,3244 2246)))`

 * After validation we have a geometry collection formed by 1 polygon and a
 multistring made of 2 lines:
 `+ SRID=3857;GEOMETRYCOLLECTION(POLYGON((3245 2224,3262 2030,3253
 2158,3245 2224)),MULTILINESTRING((3244 2244,3244 2246),(3244 2243,3244
 2244)))`

 * Since MVT is incompatible with collections types, mvt_geom drops the
 geometry with the smallest geometry type:
 `SRID=3857;MULTILINESTRING((3244 2244,3244 2246),(3244 2243,3244 2244))`

 I intend to fix it by saving the initial type and preferring something
 equivalent when extracting the basic type at the last step.

 For reference, what Mapnik does for geometry collections is to duplicate
 the features (one feature per each geometry in the collection), but this
 also has some drawbacks like duplicating what should be unique values.

-- 
Ticket URL: <https://trac.osgeo.org/postgis/ticket/4181#comment:7>
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