15.00 - Guidelines for Collecting Statistics On Single-Table Join Index Columns - Teradata Database

Teradata Database Design

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User Guide

Guidelines for Collecting Statistics On Single-Table Join Index Columns

The guidelines for selecting single-table join index columns on which to collect statistics are similar to those for base tables. 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 support 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. With this in mind, consider the following factors when deciding which columns to collect statistics for.

  • Always consider collecting statistics on the primary index. This is particularly critical for accurate cardinality estimates.

    IF an execution plan might involve …

    THEN collect statistics on the …

    search condition keys

    column set that constitutes the search condition predicate.

    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.


    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 clause predicates, 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.