The two-dimensional R-tree index differs from the traditional hierarchical (one-dimensional) B-tree index. Spatial data is two dimensional, so you cannot use the B-tree index for spatial data. Similarly, you cannot use an R-tree index with nonspatial data. The R-tree access method organizes data in a tree-shaped structure with bounding boxes at the nodes.
A search using an R-tree index descends the tree to find objects in the general area of interest and perform tests on the objects themselves. An R-tree index eliminates the need to examine objects outside the area of interest. Without an R-tree index, a query would need to evaluate every object to find those that match the query criteria.
IBM Informix uses an R-tree index to perform spatial queries. For more information about the Informix R-tree index, consult the IBM Informix Spatial DataBlade Module User's Guide. The content of this manual is available in the IBM Knowledge Center.
In PostgreSQL, the R-tree index is implemented using the Generalized Search Tree (GiST) index infrastructure. For information on GiST indexing, see the PostgreSQL documentation.