Query optimization

Now in the real world, a lot of information is stored in the relational database. People store information and use it when they need. A lot of data are used everyday. “SQL is the standard programming language for creating, manipulating, and retrieving information stored in relational databases.”(Rachel Singer Gordon, 2006) People use SQL operate database for query.
But for one SQL statement, database management system (DBMS) can follow many plans to process it and produce the result. All plans would produce the same result, but their costs are different such as the amount of time of processing. Which plan is the best choice and how we find it? Query Optimization is very necessary and important in DBMS. (Yannis E. Ioannidis)
“There has been extensive work in query optimization since the early 70s.” (Surajit Chaudhuri) Query optimization was researched for such a long time, it still need to improve as there are a lot of factors need to think about such as Search Space, I/O Cost and CUP Cost and so on. These factors may affect the performance of query such as speed of query and resource used. (John G. Hughes, 1988, p 177)
“The query optimization phase in query processing plays a crucial role in choosing the most efficient strategy for executing a query.”(David Taniar, Hui Yee Khaw, Haorianto Cokrowijoyo Tjioe & Eric Pardede, 2007)