Extensions
Scalar Functions
Collects a list of geometries into a collection geometry.  
* If all geometries are `POINT`'s, a `MULTIPOINT` is returned.
* If all geometries are `LINESTRING`'s, a `MULTILINESTRING` is returned.
* If all geometries are `POLYGON`'s, a `MULTIPOLYGON` is returned.
* Otherwise if the input collection contains a mix of geometry types, a `GEOMETRYCOLLECTION` is returned.  
Empty and `NULL` geometries are ignored. If all geometries are empty or `NULL`, a `GEOMETRYCOLLECTION EMPTY` is returned.