An Oracle view's execution plan, when the view references a table containing an St_Geometry attribute with a spatial index, is derived in the same manner as an execution plan for a SQL statement executed directly against the table itself.
The view is decomposed to the table objects it references by the optimizer. When the view contains an operator that references an St_Geometry attribute, the optimizer delegates the heuristics for calculating the queries cost to the st_domain_stats type.
The st_domain_stats type uses the statistics on the table the view references to calculate selectivity and cost. The statistics are maintained in the st_geometry_index table.