ST_M — Returns the M coordinate of a Point.
float ST_M(geometry  a_point);
Return the M coordinate of a Point, or NULL if not available. Input must be a Point.
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This is not (yet) part of the OGC spec, but is listed here to complete the point coordinate extractor function list.  | 
 This method implements the OGC Simple Features
 Implementation Specification for SQL 1.1.
 This method implements the SQL/MM specification.
 This function supports 3d and will not drop the z-index.