pgr_alphashape is returning a geometry collection?

[Dapeng Wang]

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
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