public PlanType transformExpressions(PartialFunction<Expression, Expression> rule) {
        return transformExpressionsDown(rule);
    }
Runs [[transform]] with `rule` on all expressions present in this query operator.
Users should not expect a specific directionality. If a specific directionality is needed,
transformExpressionsDown or transformExpressionsUp should be used.

@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