Basic Steps:Optimization(Cont.) Query Optimization:Amongst all equivalent evaluation plans choose the one with lowest cost. Cost is estimated using statistical information from the database catalog e.g.number of tuples in each relation,size of tuples,etc. In this chapter we study How to measure query costs Algorithms for evaluating relational algebra operations How to combine algorithms for individual operations in order to evaluate a complete expression In Chapter 14 We study how to optimize queries,that is,how to find an evaluation plan with lowest estimated cost Database System Concepts-6th Edition 12.6 ©Silberschat乜，Korth and SudarshanDatabase System Concepts - 6 12.6 ©Silberschatz, Korth and Sudarshan th Edition Basic Steps: Optimization (Cont.) Query Optimization: Amongst all equivalent evaluation plans choose the one with lowest cost. Cost is estimated using statistical information from the database catalog  e.g. number of tuples in each relation, size of tuples, etc. In this chapter we study How to measure query costs Algorithms for evaluating relational algebra operations How to combine algorithms for individual operations in order to evaluate a complete expression In Chapter 14 We study how to optimize queries, that is, how to find an evaluation plan with lowest estimated cost