The Optimizer substitutes base table statistics for single-table join index statistics when no demographics have been collected for its single-table indexes. Because of the way single-table join index columns are built, instead of collecting statistics directly on the index columns, collect statistics on the corresponding columns of the base table. This optimizes both system performance and disk storage by eliminating the need to collect the same data redundantly.
You may need to collect statistics on single-table join index columns instead of their underlying base table columns if you decide not to collect the statistics for the relevant base table columns. Collect statistics directly on the corresponding single-table join index columns.
The derived statistics framework can use bidirectional inheritance of statistics between base tables and their underlying non-sparse single-table join indexes, so the importance of collecting statistics on base table columns rather than non-sparse single-table join index columns is no longer as important as it once was (see Derived Statistics for details).