Functional R-tree indexes

Under certain conditions, you can create a functional R-tree index, using one of the several spatial data type functions. A functional index is built using the value returned by a function rather than the value of a column in a table.

For example, you can create an R-tree index on the centroids of the objects in a table, rather than on the objects themselves: 
CREATE TABLE poly_tab (poly_col ST_Geometry);

CREATE INDEX centroid_idx ON poly_tab 
   (ST_Centroid(poly_col) ST_Geometry_ops) USING RTREE;
You can use the following spatial data type functions in a functional index:
  • ST_Centroid()
  • ST_EndPoint()
  • ST_Envelope()
  • SE_Midpoint()
  • ST_Point()
  • ST_PointN()
  • ST_PointOnSurface()
  • ST_StartPoint()