The spatial grid index
ArcSDE geodatabases in Oracle and DB2 use grid indexes. The spatial index is built by applying a grid to the data in the spatial column. The spatial grid index is two-dimensional and spans a feature class, similar to the reference grid you might find on a common road map. You can assign the spatial grid index one, two, or three grid levels, each with its own distinct cell size. The mandatory first grid level has the smallest cell size. The optional second and third grid cell levels are disabled by setting them to 0. If enabled, the second grid cell size must be at least three times larger than the first grid cell size, and the third grid cell size must be three times larger than the second grid cell size.
In the following example, the feature class has two grid levels. Area shape 101 is located in grid cell 4 on level 1. A record is added to the spatial index table, because the feature resides within four grid cells (in this case, it is one). The envelope for area feature 102 is located in cells 1 through 8 on level 1. Because the feature's envelope resides in more than four grid cells, the feature is promoted to level 2, where its envelope fits within two grid cells. Feature 102 is indexed at level 2, and two records are added to the spatial index table.
Inserting, updating, or deleting a feature updates the spatial index. The extent of each feature is overlaid onto the lowest grid level to obtain the number of grid cells. If the feature exceeds the value set for the MAXGRIDSPERFEAT value in the SERVER_CONFIG table, the geometry is promoted to the next highest grid level, if you have defined one.
For Oracle databases, you can specify the location in which the spatial index is created by setting the S_STORAGE parameter of the configuration keyword you use to create the feature class. See Oracle DBTUNE configuration parameters and the sdedbtune command in the ArcSDE Administration Command Reference for information on setting configuration parameters.