[PostGIS] #6136: ST_MakeValid merges polygons in a MultiPolygon.

[PostGIS]

#6136: ST_MakeValid merges polygons in a MultiPolygon.
-------------------------------+---------------------------
 Reporter:  Lars Aksel Opsahl  |      Owner:  pramsey
     Type:  defect             |     Status:  new
 Priority:  medium             |  Milestone:  PostGIS 3.6.5
Component:  postgis            |    Version:  3.6.x
 Keywords:                     |
-------------------------------+---------------------------
 In the MULTIPOLYGON below, eight polygons contain internal intersections,
 making the geometry invalid.

 Running ST_MakeValid() removes most of the internal boundaries and reduces
 the result to only two polygons in attached file.

 Is this the expected behavior?

 If so, it might be useful to have a function such as
 ST_MakeValidPreserveTopology() that preserves the original polygon
 boundaries and structure as much as possible to produce a valid geometry.

 This was tested with:

 {{{
 POSTGIS="3.7.0dev 3.6.0rc2-473-g260127869"
 PGSQL="16.0"
 GEOS="3.14.1-CAPI-1.20.5"
 PROJ="9.8.1"
 }}}
 and on other servers with similar results.

 ST_Buffer(geom, 0) behaves almost the same way, except that it returns
 three polygons instead of two in attached file.
-- 
Ticket URL: <https://trac.osgeo.org/postgis/ticket/6136>
PostGIS <http://trac.osgeo.org/postgis/>
The PostGIS Trac is used for bug, enhancement & task tracking, a user and developer wiki, and a view into the subversion code repository of PostGIS project.