Single-table non-sparse join indexes inherit their statistics from their underlying base table if the statistics have not been collected on the index itself. This applies to single-column statistics, single-column-index statistics, multicolumn statistics, and multicolumn index statistics.
Available Forms of Statistical Inheritance
The following list summarizes the available forms of statistical inheritance:
- Underlying base tables can inherit the statistics from any single-table non-spare join indexes that reference them. Inherited statistics can come from multiple qualified join indexes.
- Multicolumn statistics are inherited from an underlying base table by a single-table non-sparse join index defined on it.
- Partitioning columns can inherit the statistics from the system-derived PARTITION column, but only if the table is partitioned by a single-column partitioning expression.
- Bidirectional inheritance of statistics is implemented for single-table non-sparse join indexes so that statistics collected on the single-table non-sparse join index are inherited by their underlying base table.
There can be multiple qualified single-table non-sparse join indexes from which a base table inherits statistics.
- Bidirectional inheritance of statistics is implemented for PARTITION statistics, where those statistics are inherited by the partitioning column of a row-partitioned table if the table is partitioned by a single-column partitioning expression.
Bidirectional inheritance of statistics is not supported for PARTITION#L n statistics.
The inheritance of statistics occurs when the derived statistics are built for the base relations. All statistical inheritance is bidirectional.