Publisher Summary This chapter presents tips and suggestions on optimizing SQL. The query optimizers depend on the underlying architecture and are simply too different for many universal rules. A rule-based optimizer parses a query and executes it in the order in which it was written, by doing some reorganization of the query into an equivalent form using some syntax rules. A cost-based optimizer looks at both the query and the statistical data about secondary storage, shares the current data in cache by using sessions, and then decides on the best way to execute the query. These decisions are based on factors such as the access methods used, the data to be brought into main storage, and the types of sorting technique used. In disk systems, there are four basic methods of getting to data that include table scans or sequential reads of all the rows in the table, access via some kind of tree index, hashing, and bit vector indexes. Writing simple search conditions is suggested as optimizers have trouble using an index with complicated expressions or function calls. As string expressions can be expensive with the LIKE predicate, the use of “%” should be avoided in favor of “_” in the pattern string. Providing extra information can also be helpful as the optimizer will be able to find an improved execution plan with more information in hand.