When a join statement selects from three or more tables, analyzes any equijoin predicates between simple column references within each query block and adds additional equijoin predicates where possible if they do not currently exist. For example, transforms the following query: SELECT * FROM samp.employee e, samp.emp_act a, samp.emp_resume r WHERE e.empno = a.empno and a.empno = r.empno

into the following: SELECT * FROM samp.employee e, samp.emp_act a, samp.emp_resume r WHERE e.empno = a.empno and a.empno = r.empno and e.empno = r.empno

On the other hand, the optimizer knows that one of these equijoin predicates is redundant and will throw out the one that is least useful for optimization.