Basic Steps in Query Processing: Optimization A relational algebra expression may have many equivalent expressions E.g.salary75000(saay(instructor))is equivalent to Πsalary(salary<75ooo(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.,: Use an index on salary to find instructors with salary 75000. Or perform complete relation scan and discard instructors with salary ≥75000 Database System Concepts-7th Edition 15.5 ©Silberscha乜，Korth and SudarshanDatabase System Concepts - 7 15.5 ©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.,: • Use an index on salary to find instructors with salary < 75000, • Or perform complete relation scan and discard instructors with salary  75000