Degenerated polygons from ST_Difference

[José de Paula Rodrigues Neto Assis]

When trying to do a ST_Difference between two fairly complex MultiPolygons, the result is a MultiPolygon composed of rectangles and triangles, looking like overly simplified shapes.

• Database version: PostgreSQL 12.3
• OS/Architecture: Gentoo Linux, AMD64 (also reproduced with CentOS 8/PGDG repo)
• PostGIS version: 3.0.1
• GEOS version: 3.8.1
• GDAL version: 3.1.0
• PROJ version: 7.0.1

The table definitions are as follows:

CREATE TABLE tb_test_a
(
    id integer PRIMARY KEY,
    geom geometry(MultiPolygon,4674)
);
CREATE TABLE tb_test_b
(
    id integer PRIMARY KEY,
    geom geometry(MultiPolygon,4674)
);

They have a single record each, with a different geometry in each case, which I'm attaching to this report; they are both ST_Valid() MultiPolygons.

This is what generates the degenerated polygons:

SELECT
	ST_Difference(a.geom, b.geom) geom
FROM
	tb_test_a a,
	tb_test_b b

If I wrap both geometries with ST_MakeValid(ST_SnapToGrid()), I get the expected results:

SELECT
	ST_Difference(
		ST_MakeValid(ST_SnapToGrid(a.geom, 0.0000000001)),
		ST_MakeValid(ST_SnapToGrid(b.geom, 0.0000000001)) ) geom
FROM
	tb_test_a a,
	tb_test_b b

I have to make ST_MakeValid after snapping because of self-intersection errors.

I've also seen cases of ST_Intersection failing the same way (generating too simplified results) with similarly complex source MultiPolygons (but not with the attached data).

[José de Paula Rodrigues Neto Assis]

PostgreSQL 12.0 database dump of the bug data

[José de Paula Rodrigues Neto Assis]

Preview of Table A geometry

[José de Paula Rodrigues Neto Assis]

Preview of Table B geometry

[José de Paula Rodrigues Neto Assis]

Preview of ST_Difference - this is the bug

[pramsey]

As of GEOS 3.9, the output is a valid polygon.