> Eliminate the table T from the query/subquery if the following requirements > are satisfied: > 1. T is left joined > 2. T is referenced only in the join expression where it is left joined > 3. the left join's join expression is a simple equality expression like > T1.C1=T2.C2; T1!=T2 and (T==T1 or T==T2) > 4. the column of T in the join exression is the primary key of T
Condition 4 should be: the column of T in the join expression is a key of T (i.e. it doesn't need to be the PK, a UNIQUE constraint would be enough).
This process can be done recursively (implementation doesn't have to be recursive, of course), to eliminate whole sub-trees of the join tree.