The function pgr_alphashape returns "GEOMETRY COLLECTION" as its result. For reference, please see https://docs.pgrouting.org/3.6/en/pgr_alphaShape.html.
In PostGIS, the function used is ST_AlphaShape, which returns a geometry. An alpha-shape is a (usually) concave polygonal geometry that contains all the vertices of the input and whose vertices are a subset of the input vertices. Please refer to https://postgis.net/docs/manual-dev/ST_AlphaShape.html. Dapeng
