How to force Firebird to use a particular query PLAN?

It's quite easy, just add a PLAN clause to your query like this:

select * from EMPLOYEE
where EMP_NO = 5
PLAN (EMPLOYEE NATURAL)

Make sure you know what you are doing and that you're sure that your plan is better. Sometimes it's enough to reset index statistics (see FAQ #110 for details) and Firebird's query optimizer will pick it up. Of course, sometimes you really don't want to use some index, see FAQ #158 for more robust way of preventing index usage (one you don't have to change when you add stuff to the query).

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