ECE356 Lecture Notes - Lecture 20: Briey, Mysql, Pseudocode

• School: University of Waterloo
• Department: Electrical and Computer Engineering
• Course: ECE356
• Professor: Zarnett

Document Summary

Lecture 20 query optimization and plan selection. The nal advanced subject in query optimization is complex enough that it deserves a more detailed look: join elimination. So much of the previous examination has focused on the cost of the join and that has highlighted in a real way just how expensive it is to perform a join. For this reason, good optimizer routines will attempt to eliminate the join altogether if it can be skipped. The optimizer can only do this if there is certainty that the outcome will not be a ected by not doing the join. We will shortly see how that is accomplished. Developers make mistakes, as you know, or perhaps some legacy code cannot be changed for some reason. Regardless, sql is a language in which you specify the result that you want, not speci cally how to get it.