Remember These Points

• The Optimizer captures statistics for skewed column values in a structure known as a histogram.
• SQL expressions or functions which are included in the predicate of the SQL text can cause the Optimizer to grossly miscalculate the selectivity of a query, effectively creating column value skews. Extended statistics collection must be employed to solve this problem.
• Interrelationships between the columns of a table can likewise result in selectivity miscalculations for multi-column SQL predicates. Collecting MultiColumn statistics will resolve this problem.
• Extended statistics directives may be permanently assigned to a table using either the CREATE_EXTENDED_STATS() procedure or the standard collection procedures from DBMS_STATS()