ST_AlphaShape

-- ST_AlphaShape returns the alpha shape geometry of the input geometry, based on a specified radius value.

SELECT ST_GeomFromText('MULTIPOINT((0 0), (0 1), (1 1), (.3 .7))', 0).ST_AlphaShape(1.5).ST_AsText() FROM dummy;
SELECT ST_GeomFromText('MULTIPOINT((0 0), (0 1), (1 1), (.3 .7))', 0).ST_AlphaShape(0.6).ST_AsText() FROM dummy;
SELECT ST_GeomFromText('MULTIPOINT((0 0), (0 1), (1 1), (.3 .7))', 0).ST_AlphaShape(0.3).ST_AsText() FROM dummy;
SELECT ST_GeomFromText('MULTIPOINT((0 0), (0 1), (1 1), (.3 .7))', 0).ST_AlphaShape(0.1).ST_AsText() FROM dummy;

-- returns NULL when an empty geometry found in input
SELECT ST_GeomFromText('POLYGON EMPTY', 1000004326).ST_AlphaShape(3.0, 'meter') FROM dummy;

-- error due to using round earth system (4326)
SELECT ST_GeomFromText('POLYGON EMPTY', 4326).ST_AlphaShape(3.0, 'meter') FROM dummy;