ST_Simplify(g, max_distance)

Simplifies a geometry using the Douglas-Peucker algorithm and returns a
simplified value of the same type. If any argument is NULL, the return
value is NULL.

The geometry may be any geometry type, although the Douglas-Peucker
algorithm may not actually process every type. A geometry collection is
processed by giving its components one by one to the simplification
algorithm, and the returned geometries are put into a geometry
collection as result.

The max_distance argument is the distance (in units of the input
coordinates) of a vertex to other segments to be removed. Vertices
within this distance of the simplified linestring are removed. If the
max_distance argument is not positive, or is NaN, an ER_WRONG_ARGUMENTS
error occurs.

According to Boost.Geometry, geometries might become invalid as a
result of the simplification process, and the process might create
self-intersections. To check the validity of the result, pass it to
ST_IsValid().

If the geometry argument is not a syntactically well-formed geometry
byte string, an ER_GIS_INVALID_DATA error occurs.

URL: http://dev.mysql.com/doc/refman/5.7/en/spatial-convenience-functions.html

