自己实现数据库（4）优化器

public double estimateJoinCost(LogicalJoinNode j, int card1, int card2,
            double cost1, double cost2) {
        if (j instanceof LogicalSubplanJoinNode) {
           
            return card1 + cost1 + cost2;
        } else {
            // TODO IMPORTANT compare IO cost with CPU cost
            /*
             *   joincost(t1 join t2) = scancost(t1) + ntups(t1) x scancost(t2) //IO cost
             *                          + ntups(t1) x ntups(t2)  //CPU cost
             */
            // Insert your code here.
            // HINT: You may need to use the variable "j" if you implemented
            // a join algorithm that's more complicated than a basic
            // nested-loops join.
            return cost1 + card1 * cost2 + card1 * card2;
        }
    }