PostGIS: Ensure Intersection of 2 Polygons results in a Polygon

Question

I want to ensure that the result of a ST_Intersection(polygon1, polygon2) is also a polygon. I initially started by filtering with a ST_Overlaps(polygon1, polygon2) but that seems to only ensure that both features intersect while neither feature contains the other.

Besides filtering by checking the result of the intersection operation e.g. ST_Dimension(ST_Intersection(polygon1, polygon2)) > 1 is there a way to filter polygons by ensuring that they share some internal space that is not a boundary?

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ST_Crosses(polygon1, polygon2) actually does perform the filtering that I want.

Answer 1

The simplest way is to use ST_GeometryType. In your case, to check for polygons only, you can do,

SELECT ST_Intersection(polygon1, polygon2) FROM ..... 
WHERE ST_GeometryType(ST_Intersection(polygon1, polygon2)) = 'ST_Polygon';

For more complicated scenarios, you could also look at ST_Relate and the DE-9IM model, but I think that is probably overkill for what you need.

EDIT: To avoid intersecting polygon A with polygon B and polygon B with polygon A in a large table, and still ensuring that you only end up with a polygon as the intersection, you can do:

SELECT ST_Intersection(a.geom, b.geom) 
FROM some_table a, some_table b
WHERE ST_GeometryType(ST_Intersection(a.geom, b.geom)) = 'ST_Polygon'
AND a.id > b.id;

assuming you have a primary/unique key called id.