Expression transform(Expression e) {
            Expression newE = e.transformDown(rule);
            if (e.fastEquals(newE)) {
                return e;
            } else {
                changed = true;
                return newE;
            }
        }
Runs [[transformDown]] with `rule` on all expressions present in this query operator.

@param rule the rule to be applied to every expression in this operator.
indexr-query-opt/src/main/java/io/indexr/query/plan/QueryPlan.java