Returns true if expr can be evaluated using only the output of plan.
Returns true if expr can be evaluated using only the output of plan. This method
can be used to determine when it is acceptable to move expression evaluation within a query
plan.
For example consider a join between two relations R(a, b) and S(c, d).
canEvaluate(EqualTo(a,b), R) returns true where as canEvaluate(EqualTo(a,c), R) returns
false.