The convex hull of a geometry is the smallest convex geometry that contains all of the points in the geometry.
The convex hull may be visualized by imagining an elastic band stretched to enclose all of the points in the geometry.
When released, the elastic band takes the shape of the convex hull.
If the geometry consists of a single point, the point is returned.
If all of the points of the geometry lie on a single straight line segment, a linestring is returned.
Otherwise, a convex polygon is returned.
The convex hull can serve as an approximation of the original geometry. When testing a spatial
relationship, the convex hull can serve as a quick pre-filter because if there is no spatial
intersection with the convex hull then there can be no intersection with the original geometry.
If the geometry-expression is an empty geometry (ST_IsEmpty()=1), then this method returns NULL.
ST_ConvexHull is not supported on geometries which contain circular strings.
This method can not be used with geometries in round-Earth spatial reference system.