R-trees have been already integrated into commercial DBMSs. The following (obviously, non-exhaustive) list provides relevant information:

	Oracle: Datasheet on Oracle 9i rel.2 Spatial, white paper on Oracle Spatial (with more details on R-tree implementation)
	IBM Informix: White paper, R-tree description
	MySQL: GIS and spatial extensions with MySQL
	PostgreSQL: Interfacing extensions to indexes (R-tree has been re-implemented using GiST in rel. 82)