ST_Geometry::ST_ConvexHullAggr(geometry-column)

The ST_ConvexHullAggr considers all of the points in the group of geometries it is computed over and returns the convex hull of all these points. 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 geometries in the group consist of a single point, the point is returned. If all of the points of the group of geometries 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.

• ST_ConvexHull method for type ST_Geometry

The following example returns the result Polygon ((3 0, 7 2, 3 6, 0 7, -3 6, -3 3, 0 0, 3 0)).

SELECT ST_Geometry::ST_ConvexHullAggr( Shape ) 
FROM SpatialShapes WHERE ShapeID <= 16