After a query is parsed and preprocessed, but before the query optimizer begins its plan analysis, the query is transformed to increase the number of clauses that can be optimized. The transformation changes made by the optimizer are transparent unless the output of such query tuning tools as showplan, dbcc(200), statistics io, or the set commands is examined. If you run queries that benefit from the addition of optimized search arguments, the added clauses are visible. In showplan output, it appears as “Keys are” messages for tables for which you specify no search argument or join.
