query optimizations
which of these rules are valid?
- filter C1 from filter C2 from T = filter C1 and C2 from T
- select COLS1 from select COLS2 from T = select COLS1 U COLS2 from T
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under what conditions are these valid?
- select COLS from (join of T1 and T2) = join (select COLS from T1) and (select COLS from T2)
- filter C from (join of T1 and T2) = join of (select C from T1) and T2