Basic Steps in Query Processing Optimization A relational algebra expression may have many equivalent expressions E.g.,salary<75000(Isalary(instructor)is equivalent to IIsalary (Osalary<75000(instructor) Each relational algebra operation can be evaluated using one of several different algorithms Correspondingly,a relational-algebra expression can be evaluated in many ways. Annotated expression specifying detailed evaluation strategy is called an evaluation-plan. E.g.,can use an index on salary to find instructors with salary 75000, or can perform complete relation scan and discard instructors with salary≥75000 Database System Concepts-6th Edition 12.5 ©Silberschat乜，Korth and SudarshanDatabase System Concepts - 6 12.5 ©Silberschatz, Korth and Sudarshan th Edition Basic Steps in Query Processing : Optimization A relational algebra expression may have many equivalent expressions E.g., salary75000(salary(instructor)) is equivalent to salary(salary75000(instructor)) Each relational algebra operation can be evaluated using one of several different algorithms Correspondingly, a relational-algebra expression can be evaluated in many ways. Annotated expression specifying detailed evaluation strategy is called an evaluation-plan. E.g., can use an index on salary to find instructors with salary < 75000, or can perform complete relation scan and discard instructors with salary  75000