Schema overrides are provider-specific because different FDO providers support FDO data stores with widely different physical formats. Therefore, the types of schema mappings in these overrides also vary between providers. For example, an RDBMS-type provider might provide a mapping to index a set of columns in a class table. However, other providers would not necessarily be able to work with the concept of an index. For information about schema overrides support by a specific provider, see the appropriate appendix in this document and The Essential FDO.