ST_BoundingDiagonal — Returns the diagonal of a geometry's bounding box.
geometry ST_BoundingDiagonal(geometry geom, boolean fits=false
Returns the diagonal of the supplied geometry's bounding box as a LineString. If the input geometry is empty, the diagonal line is also empty, otherwise it is a 2-point LineString with the minimum values of each dimension in its start point and the maximum values in its end point.
The returned linestring geometry always retains SRID and dimensionality (Z and M presence) of the input geometry.
fits parameter specifies if the best fit is needed.
If false, the diagonal of a somewhat larger bounding box can be accepted
(is faster to obtain for geometries with a lot of vertices). In any case
the bounding box of the returned diagonal line always covers the input
In degenerate cases (a single vertex in input) the returned linestring will be topologically invalid (no interior). This does not make the return semantically invalid.
This function supports 3d and will not drop the z-index.
This function supports M coordinates.
-- Get the minimum X in a buffer around a point SELECT ST_X(ST_StartPoint(ST_BoundingDiagonal( ST_Buffer(ST_MakePoint(0,0),10) ))); st_x ------ -10