ECE356 Lecture Notes - Lecture 19: Mcgraw-Hill Education, Population Pyramid, Relational Algebra
26 Jul 2018
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Document Summary
Given an understanding of the various rules, there are now some clear ways we can transform an input query to determine which of the equivalent representations will have the lowest cost of execution. Suppose our query involves a selection and a join: we want to select the employee number, salary, and address for an employee with an id of 385. Suppose number and salary are in the employee table with 300 entries, and the address information is in another table with 12000 entries. We have a join query, and if we do this badly, we will compute the join of employees and addresses, producing some 300 results, and then we need to do a selection and a projection on that intermediate relation. If done e ciently, we will do the selection and projection rst, meaning the join needs to match exactly one tuple of employees rather than all 300.
