Make the best use of what is in your power, and take the rest as it happens.
Every single SQL statement sent to the database before being processed by the SQL engine must be turned into an execution plan. In fact, an application specifies only what data must be processed through SQL statements, not how to process it. The aim of the query optimizer is not only to deliver the SQL engine execution plans describing how to process data but also, and most important, to deliver efficient execution plans. Failing to do so may lead to abysmal performance. Precisely for this reason, a book about database performance must deal with the query optimizer.
The aim of this part, however, is not to cover the internal workings ...