(V.2.0.x) A PLAN clause optionally allows you to provide your own instructions to the engine and have it ignore the plan supplied by the optimizer. Firebird 2 enhancements allow you to specify more possible paths for the engine. For example:

   PLAN (A ORDER IDX1 INDEX (IDX2, IDX3))
    

For more details, please refer to the topic Query Plans Improvements in the DML chapter.