org.datanucleus.metadata.IndexMetaData
For schema generation, it might be useful to specify that a column or columns be indexed, and to provide the name of the index. For this purpose, an index element can be contained within a field, element, key, value, or join element, and this indicates that the column(s) associated with the referenced element should be indexed. Indexes can also be specified at the class level, by including index elements containing column elements. In this case, the column elements are mapped elsewhere, and the column elements contain only the column name.