SQL Server’s query optimization process is responsible for producing a query execution plan when a SELECT query is executed. Typically SQL Server will choose an efficient plan over an inefficient one. When this doesn’t happen, you will want to examine the query execution plan, table statistics, supporting indexes, and other factors that are discussed in more detail in Chapters 21 and 23. Ultimately, after researching the query’s performance, you may decide to override the decision-making process of the SQL Server query optimizer by using hints.
Caution You should almost always let SQL Server’s query optimization ...