Selections Using Indices A4(secondary index,equality on nonkey). Retrieve a single record if the search-key is a candidate key Cost=(h;+1)*(t+ts) Retrieve multiple records if search-key is not a candidate key each of n matching records may be on a different block Cost=(h;+n)*(t+ts) Can be very expensive！ Database System Concepts-6th Edition 12.12 ©Silberschat乜，Korth and SudarshanDatabase System Concepts - 6 12.12 ©Silberschatz, Korth and Sudarshan th Edition Selections Using Indices A4 (secondary index, equality on nonkey). Retrieve a single record if the search-key is a candidate key  Cost = (hi + 1) * (tT + tS) Retrieve multiple records if search-key is not a candidate key  each of n matching records may be on a different block  Cost = (hi + n) * (tT + tS) – Can be very expensive!