Teradata SQL supplies a very powerful EXPLAIN request modifier that allows you to see the execution plan of a query.
The EXPLAIN request modifier not only explains how a request will be processed but also provides an estimate of the number of rows involved as well as the performance impact of the request. Teradata Database supports EXPLAIN request modifiers with detailed Optimizer information including, for example, cost estimates for Insert, Update, Upsert, Merge, and Delete steps, as well as spool size estimates.
EXPLAIN helps you to evaluate complex queries and develop alternative, more efficient, processing strategies. You may be able to get a better plan by collecting more statistics on more columns, or by defining additional indexes. Your knowledge of the actual demographics information may allow you to identify row count estimates that seem badly wrong, and help to pinpoint areas where additional statistics would be helpful.