In optimization, where should you use the primary key field in a query against data extensions?

• A. Do not use the primary key
• B. Use the primary key field in your select statement or in any join as a predicate
• C. Only use the primary key in the WHERE clause
• D. Use the primary key as a column alias

Answer: (B)

Indexed primary keys enable fast lookups in data extensions. Because a primary key uniquely identifies each row, the query engine can use the index to locate matches quickly. Including the primary key in the select list helps when you need to return or verify that unique rows are being retrieved, and using it as a predicate in a join (the ON condition) lets the engine perform efficient, indexed joins rather than scanning the entire extension. This combination gives the best performance for both filtering and joining. Using the key only in a WHERE clause misses some of the join-optimization benefits, and using it as a column alias does not affect how data is retrieved.