Introduction(Cont.) Cost difference between evaluation plans for a query can be enormous E.g.seconds vs.days in some cases Steps in cost-based query optimization 1.Generate logically equivalent expressions using equivalence rules 2.Annotate resultant expressions to get alternative query plans 3.Choose the cheapest plan based on estimated cost Estimation of plan cost based on: Statistical information about relations.Examples: number of tuples,number of distinct values for an attribute Statistics estimation for intermediate results to compute cost of complex expressions Cost formulae for algorithms,computed using statistics Database System Concepts-6th Edition 1.5 ©Silberschat乜，Korth and SudarshanDatabase System Concepts - 6 1.5 ©Silberschatz, Korth and Sudarshan th Edition Introduction (Cont.) Cost difference between evaluation plans for a query can be enormous E.g. seconds vs. days in some cases Steps in cost-based query optimization 1. Generate logically equivalent expressions using equivalence rules 2. Annotate resultant expressions to get alternative query plans 3. Choose the cheapest plan based on estimated cost Estimation of plan cost based on: Statistical information about relations. Examples:  number of tuples, number of distinct values for an attribute Statistics estimation for intermediate results  to compute cost of complex expressions Cost formulae for algorithms, computed using statistics