Primary Index (fastest) - When ever a Primary Index is utilized in the SQL WHERE Clause the PE will be able to use the Primary Index to get the data with a one- AMP operation.
Secondary Index (next fastest) - If the Primary Index is not utilized sometimes Teradata can utilize a secondary index. It is not as fast as the Primary Index, but it is much faster than a full table scan.
Full Table Scan (FTS) (Slowest) Teradata handles full table scans brilliantly because Teradata accesses each data row only once because of the parallel processing. Full Table Scans are a way to access Teradata without using an index. Each data block per table is read only once.