Spatial indexing technology is based on the traditional hierarchical B tree index, but is significantly different. The spatial index utilizes grid indexing which is designed to index two-dimensional spatial columns. The B tree index can only handle one-dimensional data and cannot be used with GIS information. This section describes how a B tree index is structured and used.
The top level of a B tree index, called the root node, contains one key for each node at the next level. The value of each key is the largest existing key value for the corresponding node at the next level. Depending on the number of values in the base table, several intermediate nodes may be needed. These nodes form a bridge between the root node and the leaf nodes that hold the actual base table row IDs.
The database manager searches a B tree index starting at the root node. It then continues through the intermediate nodes until it reaches the leaf node with the row ID of the base table.
The B tree index cannot be applied to a spatial column because the two-dimensional characteristic of the spatial column requires the structure of a spatial index. For the same reason, you cannot apply a spatial index to a non-spatial column. Further, a spatial index cannot be applied to a composite column of any kind.