In relation to this question here as the image below.
I have a multipolygon geometry (red lines) that i want to turn into lines so i can use it to split another multipoint polygon (black blocks) using ST_Split. enter image description here.

Is there a function equivalent to ST_ExteriorRing that works on multipolygons?
I've tried:

SELECT ST_Split(bldgs.geom, ST_Boundary(admin_boundary))
FROM admin_boundary, bldgs
WHERE admin_boundary.borders = 'N4 2';

ERROR: Splitting a Polygon by a MultiLineString is unsupported

ST_ExteriorRing can't be used because it won't work on multipolygons

2 Answers 2


Yes, you have to use ST_Dump

From the manual:

ST_Dump is useful for expanding geometries. It is the reverse of a GROUP BY in that it creates new rows. For example it can be use to expand MULTIPOLYGONS into POLYGONS.

In your case the query would likely look like:

WITH polygons AS (
  SELECT (ST_Dump(geom)).geom 
  FROM admin_boundary
  WHERE admin_boundary.borders = 'N4 2'
 ST_Split(bldgs.geom, admin_boundary) geom
FROM bldgs a 
LEFT JOIN polygons b ON ST_Intersects(a.geom, b.geom)
  • How does that work in the above context? Aug 16, 2017 at 14:44
  • I've added an example. While checking the docs, I noticed though that cutting with multipolygons shouldn't be a problem. From the ST_Split documentation: When a (multi)polygon is passed as as the blade, its linear component (the boundary) is used for cutting the input.
    – tilt
    Aug 16, 2017 at 14:59
  • Yes, i re-read that and was part of my problem in the above both are multipolygons so the black building blockslayer needs to be turned into lines before being passed to ST_Split Aug 16, 2017 at 15:15
  • Then I would advise you to update your question, or even put a new one. Because the current question and answer don't make sense in that case.
    – tilt
    Aug 17, 2017 at 9:45

You can use the create a custom function ST_SplitPolygon as written here. This function allows a multipolygon to be cut by a multline blade

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