You mean adding additional key columns to the index. Adding nonkey columns to the index puts them at the leaf node so the engine doesn't have to do a key lookup

You mean adding additional key columns to the index. Adding nonkey columns to the index puts them at the leaf node so the engine doesn't have to do a key lookup back to the main table to get all the additional columns from your query. More details on covering indexes here: https://www.simple-talk.com/sql/learn-sql-server/using-covering-indexes-to-improve-query-performance/ https://www.simple-talk.com/sql/learn-sql-server/using-cover...