ST_ConcaveHull of MultiPolygon returns disconnected result

[nbvfgh]

Considering following MultiPolygon:

SELECT ST_GeomFromText('MULTIPOLYGON (((0 0, 1 0, 2 1, 1 1, 0 0)), 
					((8 2, 8 4, 10 4, 10 2, 8 2)),
					((4 9, 5 9, 5 8, 4 8, 4 9)))');

We calculate its concave shell with ST_ConcaveHull:

ST_ConcaveHull(geom, param_pctconvex, param_allow_holes)：
As described in the document, the param_pctconvex controls the concaveness of the computed hull. A value of 0 produces a hull with maximum concaveness (but still a single polygon). And often values between 0.3 and 0.1 produce reasonable results.

CREATE TABLE t1(geom geometry);

INSERT INTO t1 VALUES(ST_GeomFromText('MULTIPOLYGON (((0 0, 1 0, 2 1, 1 1, 0 0),(0 0, 1 1, 2 1, 0 0)), 
		                      ((8 2, 8 4, 10 4, 10 2, 8 2)),
		                      ((4 9, 5 9, 5 8, 4 8, 4 9)))');

SELECT ST_AsText(ST_ConcaveHull(t1, 0)) from t1;

-- expected: { POLYGON(...)}
-- actual  : { MULTIPOLYGON(((0 0,1 0,2 1,1 1,0 0)),((8 2,8 4,10 4,10 2,8 2)),((4 9,5 9,5 8,4 8,4 9)))}

The result of concavehull degenerates into the original input

[nbvfgh]

bug tracker: libgeos/geos#1212