Use EXPLAIN or Visual Explain to Analyze the Usefulness of Secondary Indexes
Before a request is processed, the Optimizer estimates comparative costs to determine whether the use of a secondary index offers savings or is plausible. If not, it uses a different access method. It is advisable, therefore, to test the performance of a secondary index before implementing it.
You can use either the SQL EXPLAIN request modifier (see SQL Data Manipulation Language) or the Visual Explain client utility (see Teradata Visual Explain User Guide) to generate a description of the processing sequence to help determine whether a secondary index should be retained.