The guidelines for selecting single-table join index columns on which to collect statistics are similar to those for base tables and hash indexes. The primary factor to consider in all cases is whether the statistics provide better access plans. If they do not, consider dropping them. If the statistics you collect produce worse access plans, then you should always report the incident to Teradata Services personnel.
When you are considering collecting statistics for a single-table join index, it might help to think of the index as a special kind of base table that stores a derived result. For example, any access plan that uses a single-table join index must access it with a direct probe, a full table scan, or a range scan.
- Always consider collecting statistics on the primary index if the join index has one. This is particularly critical for accurate cardinality estimates.
- Consult the following table for execution plan cases that suggest collecting specific statistics.
IF an execution plan might involve … THEN collect statistics on the … search condition keys column set that constitutes the search condition. joining the single-table index with another table join columns to provide the Optimizer with the information it needs to best estimate the cardinalities of the join.
- Consult the following table for single-table join index cases that suggest collecting specific statistics.
IF a single-table join index is defined … THEN you should collect statistics on the … with an ORDER BY clause order key specified by that clause. without an ORDER BY clause and the order key column set from the base table is not included in the column_name_1 list order key of the base table on which the index is defined.
This action provides the Optimizer with several essential baseline statistics.
- If a single-table join index column appears frequently in WHERE clauses, you should consider collecting statistics on it as well, particularly if that column is the sort key for a value-ordered single-table join index.
- If a single-table join index is defined with a complex expression in its select list that is also specified in a predicate expression written on a mapped base table column, collecting join index statistics on the expression enhances the ability of the Optimizer to estimate single-table cardinalities for a query that specifies the expression on the base table because it can use those statistics directly to estimate the selectivity of complex expressions on base table columns specified in the query (see Teradata Vantage™ - SQL Request and Transaction Processing, B035-1142 for details).