Selections Using Indices A4(secondary index,equality on key/non-key). Retrieve a single record if the search-key is a candidate key Cost=(hi+1)*(tr+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)*(fr+ts) ·Can be very expensive！ Database System Concepts-7th Edition 15.12 ©Silberscha乜，Korth and SudarshanDatabase System Concepts - 7 15.12 ©Silberschatz, Korth and Sudarshan th Edition Selections Using Indices ▪ A4 (secondary index, equality on key/non-key). • 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!