Why is my query that uses IN or NOT IN slow?

This is an issue with Firebird optimizer. In newer Firebird versions the problem with IN is fixed, but the problem with NOT IN remains. In any case, it is safe and often faster to use EXISTS and NOT EXISTS instead. Each IN and NOT IN query can be rewritten to use EXISTS and NOT EXISTS.

Example:

select * from employee e
where e.emp_no in (select s.emp_no from SALARY_HISTORY s);

-- rewritten with EXISTS:

select * from employee e
where exists (select 1 from SALARY_HISTORY s where e.emp_no = s.emp_no);

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