[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:  assigned
 Priority:  medium      |  Milestone:  PostGIS 2.4.5
Component:  postgis     |    Version:  2.4.x
 Keywords:              |
------------------------+---------------------------
 To comply with MVT spec (2), St_AsMVTGeom forces polygon validation using
 `lwgeom_make_valid` and setting the winding order properly.

 The problem with this setup is that some polygons, under certain bboxes,
 can be transformed to lines during validation:
 {{{
 SELECT ST_AsEWKT(ST_AsMVTGeom(
     'SRID=3857;MULTIPOLYGON(((-8238034.11943289
 4970934.90416362,-8238026.20761928 4970930.11708391,-8238049.70095916
 4970891.1459086,-8238058.64331211 4970896.5565545,-8238035.14999485
 4970935.52777282,-8238034.11943289 4970934.90416362)))'::geometry,
     'SRID=3857;POLYGON((-8242969.13027341
 4975133.29702555,-8242969.13027341 4970241.3272153,-8238077.16046316
 4970241.3272153,-8238077.16046316 4975133.29702555,-8242969.13027341
 4975133.29702555))'::geometry,
     4096,
     16,
     true
 )) as the_geom_webmercator;
                           the_geom_webmercator
 ------------------------------------------------------------------------
  SRID=3857;MULTILINESTRING((4112 3546,4112 3547),(4112 3548,4112 3547))
 }}}

 In this case, after transforming the polygon to MVT coordinates in only
 has 3 points left, so make_valid transforms it into 2 lines. I've tried to
 encode a similar polygon using Mapnik and it's simply dropped out of the
 MVT.

 Although this type change isn't against the spec, it causes issues in some
 MVT rendereres as they don't expect mixed types or a geometry switching
 types depending on the zoom level.

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