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. You should test the performance of a secondary index before implementing it.
You can use either the SQL EXPLAIN request modifier or the Visual Explain client utility to generate a description of the processing sequence to help determine whether a secondary index should be retained. See Teradata Vantage™ - SQL Data Manipulation Language, B035-1146 and Teradata® Visual Explain User Guide, B035-2504.