What is spatial indexing and which indexing option should I be using?
A spatial index provides a mechanism to limit searches and speed up spatial queries, as well as allowing users to query on a specific geographic area. Its function is really quite similar to that of any other index in Oracle, only the data being indexed is of a spatial nature. Oracle allows you to select either R-Tree indexing or quadtree indexing (Fixed or Hybrid). R-Tree indexing is the default for Release 8.1.7 and 9i, but the decision of which indexing method is most appropriate for a layer is dependent on your data. The Oracle Spatial Users Guide goes into great depth describing indexing concepts and advising on when to use a given index type.
