When using query hints, you are virtually assured that the query optimizer will go through Full Optimization. The optimizer might ignore hints for a variety of reasons. For example, QUERYTRACEON trace_flag at the individual query level won’t return an error or warning if an unsupported trace flag number is used. In the same way, if the specified trace flag is not one that affects a query execution plan, the option will be silently ignored. In some cases, you’ll get an error 8622, but in many other places SQL Server will simply ignore a malformed hint. An example of this happens when directing a query to use an index that doesn’t satisfy any of the search arguments.