004--Explain實戰詳解：id分析

1.id相同代表：執行順序由上到下

mysql> EXPLAIN SELECT t1.id from t1,t3,t2  where t1.id = t3.id and t3.id = t2.id and t2.id = 1;
+----+-------------+-------+------+---------------+------+---------+------+------+--------------------------------+
| id | select_type | table | type | possible_keys | key  | key_len | ref  | rows | Extra                          |
+----+-------------+-------+------+---------------+------+---------+------+------+--------------------------------+
|  1 | SIMPLE      | t1    | ALL  | NULL          | NULL | NULL    | NULL |    3 | Using where                    |
|  1 | SIMPLE      | t3    | ALL  | NULL          | NULL | NULL    | NULL |    3 | Using where; Using join buffer |
|  1 | SIMPLE      | t2    | ALL  | NULL          | NULL | NULL    | NULL |    3 | Using where; Using join buffer |
+----+-------------+-------+------+---------------+------+---------+------+------+--------------------------------+

mysql> EXPLAIN SELECT t1.id from t1,t2,t3  where t1.id = t3.id and t3.id = t2.id and t2.id = 1;
+----+-------------+-------+------+---------------+------+---------+------+------+--------------------------------+
| id | select_type | table | type | possible_keys | key  | key_len | ref  | rows | Extra                          |
+----+-------------+-------+------+---------------+------+---------+------+------+--------------------------------+
|  1 | SIMPLE      | t1    | ALL  | NULL          | NULL | NULL    | NULL |    3 | Using where                    |
|  1 | SIMPLE      | t2    | ALL  | NULL          | NULL | NULL    | NULL |    3 | Using where; Using join buffer |
|  1 | SIMPLE      | t3    | ALL  | NULL          | NULL | NULL    | NULL |    3 | Using where; Using join buffer |
+----+-------------+-------+------+---------------+------+---------+------+------+--------------------------------+

mysql> EXPLAIN SELECT * from t1,t2,t3 where t1.id = t2.id;
+----+-------------+-------+--------+---------------+---------+---------+--------------+------+-------------------+
| id | select_type | table | type   | possible_keys | key     | key_len | ref          | rows | Extra             |
+----+-------------+-------+--------+---------------+---------+---------+--------------+------+-------------------+
|  1 | SIMPLE      | t2    | ALL    | PRIMARY       | NULL    | NULL    | NULL         |    1 |                   |
|  1 | SIMPLE      | t1    | eq_ref | PRIMARY       | PRIMARY | 4       | mysql1.t2.id |    1 |                   |
|  1 | SIMPLE      | t3    | ALL    | NULL          | NULL    | NULL    | NULL         |    3 | Using join buffer |
+----+-------------+-------+--------+---------------+---------+---------+--------------+------+-------------------+

mysql> EXPLAIN SELECT * from t1,t2 where t1.id = t2.id;
+----+-------------+-------+--------+---------------+---------+---------+--------------+------+-------+
| id | select_type | table | type   | possible_keys | key     | key_len | ref          | rows | Extra |
+----+-------------+-------+--------+---------------+---------+---------+--------------+------+-------+
|  1 | SIMPLE      | t2    | ALL    | PRIMARY       | NULL    | NULL    | NULL         |    1 |       |
|  1 | SIMPLE      | t1    | eq_ref | PRIMARY       | PRIMARY | 4       | mysql1.t2.id |    1 |       |
+----+-------------+-------+--------+---------------+---------+---------+--------------+------+-------+

mysql> EXPLAIN SELECT * from t1,t2 where t2.id = t1.id;
+----+-------------+-------+--------+---------------+---------+---------+--------------+------+-------+
| id | select_type | table | type   | possible_keys | key     | key_len | ref          | rows | Extra |
+----+-------------+-------+--------+---------------+---------+---------+--------------+------+-------+
|  1 | SIMPLE      | t2    | ALL    | PRIMARY       | NULL    | NULL    | NULL         |    1 |       |
|  1 | SIMPLE      | t1    | eq_ref | PRIMARY       | PRIMARY | 4       | mysql1.t2.id |    1 |       |
+----+-------------+-------+--------+---------------+---------+---------+--------------+------+-------+

2.id不同代表：如果是子查詢，id越大的越先被執行，理解轉化為數學運算：1+（1*(1+1)）

mysql> EXPLAIN SELECT t2.`name` from t2 where t2.id =(SELECT t1.id from t1 where t1.`name`= 'downeyjr_1');
+----+-------------+-------+------+---------------+------+---------+------+------+-------------+
| id | select_type | table | type | possible_keys | key  | key_len | ref  | rows | Extra       |
+----+-------------+-------+------+---------------+------+---------+------+------+-------------+
|  1 | PRIMARY     | t2    | ALL  | NULL          | NULL | NULL    | NULL |    3 | Using where |
|  2 | SUBQUERY    | t1    | ALL  | NULL          | NULL | NULL    | NULL |    3 | Using where |
+----+-------------+-------+------+---------------+------+---------+------+------+-------------+

mysql> EXPLAIN SELECT t2.`name` from t2 where t2.id =(SELECT t1.id from t1 where t1.`name`= (SELECT t3.`name` from t3 where t3.id= 2));
+----+-------------+-------+------+---------------+------+---------+------+------+-------------+
| id | select_type | table | type | possible_keys | key  | key_len | ref  | rows | Extra       |
+----+-------------+-------+------+---------------+------+---------+------+------+-------------+
|  1 | PRIMARY     | t2    | ALL  | NULL          | NULL | NULL    | NULL |    3 | Using where |
|  2 | SUBQUERY    | t1    | ALL  | NULL          | NULL | NULL    | NULL |    3 | Using where |
|  3 | SUBQUERY    | t3    | ALL  | NULL          | NULL | NULL    | NULL |    3 | Using where |
+----+-------------+-------+------+---------------+------+---------+------+------+-------------+

3.id相同又不同代表：先執行id值大的，然后順序執行id相同的

mysql> EXPLAIN SELECT t2.`name` from t2 ,(SELECT t3.* from t3 where t3.id= 2) t4 where t4.id = t2.id;
+----+-------------+------------+--------+---------------+------+---------+------+------+-------------+
| id | select_type | table      | type   | possible_keys | key  | key_len | ref  | rows | Extra       |
+----+-------------+------------+--------+---------------+------+---------+------+------+-------------+
|  1 | PRIMARY     | <derived2> | system | NULL          | NULL | NULL    | NULL |    1 |             |
|  1 | PRIMARY     | t2         | ALL    | NULL          | NULL | NULL    | NULL |    3 | Using where |
|  2 | DERIVED     | t3         | ALL    | NULL          | NULL | NULL    | NULL |    3 | Using where |
+----+-------------+------------+--------+---------------+------+---------+------+------+-------------+

mysql> EXPLAIN SELECT t4.`name` from (SELECT t3.* from t3 where t3.id= 2)t4,(SELECT t2.* from t2 where t2.id= 2)t5 where t4.id = t5.id ;
+----+-------------+------------+--------+---------------+------+---------+------+------+-------------+
| id | select_type | table      | type   | possible_keys | key  | key_len | ref  | rows | Extra       |
+----+-------------+------------+--------+---------------+------+---------+------+------+-------------+
|  1 | PRIMARY     | <derived2> | system | NULL          | NULL | NULL    | NULL |    1 |             |
|  1 | PRIMARY     | <derived3> | system | NULL          | NULL | NULL    | NULL |    1 |             |
|  3 | DERIVED     | t2         | ALL    | NULL          | NULL | NULL    | NULL |    3 | Using where |
|  2 | DERIVED     | t3         | ALL    | NULL          | NULL | NULL    | NULL |    3 | Using where |
+----+-------------+------------+--------+---------------+------+---------+------+------+-------------+

4.id使用總結：

  • id意義：MySQL Query Optimizer 選定的執行計劃中查詢的序列號。表示查詢中執行 select 子句或操作表的順序,id 值越大優先級越高,越先被執行。id 相同,執行順序由上至下。
  • id作用：表的讀取順序，小表驅動大表時的微調

5.執行的SQL文件

SET FOREIGN_KEY_CHECKS=0;

-- ----------------------------
-- Table structure for t1
-- ----------------------------
DROP TABLE IF EXISTS `t1`;
CREATE TABLE `t1` (
  `id` int(5) DEFAULT NULL,
  `name` varchar(10) DEFAULT NULL
) ENGINE=InnoDB DEFAULT CHARSET=utf8;

-- ----------------------------
-- Records of t1
-- ----------------------------
INSERT INTO `t1` VALUES ('1', 'downeyjr_1');
INSERT INTO `t1` VALUES ('2', 'downeyjr_2');
INSERT INTO `t1` VALUES ('3', 'downeyjr_3');

-- ----------------------------
-- Table structure for t2
-- ----------------------------
DROP TABLE IF EXISTS `t2`;
CREATE TABLE `t2` (
  `id` int(5) DEFAULT NULL,
  `name` varchar(10) DEFAULT NULL
) ENGINE=InnoDB DEFAULT CHARSET=utf8;

-- ----------------------------
-- Records of t2
-- ----------------------------
INSERT INTO `t2` VALUES ('1', 'downeyjr_1');
INSERT INTO `t2` VALUES ('2', 'downeyjr_2');
INSERT INTO `t2` VALUES ('3', 'downeyjr_3');

-- ----------------------------
-- Table structure for t3
-- ----------------------------
DROP TABLE IF EXISTS `t3`;
CREATE TABLE `t3` (
  `id` int(5) DEFAULT NULL,
  `name` varchar(10) DEFAULT NULL
) ENGINE=InnoDB DEFAULT CHARSET=utf8;

-- ----------------------------
-- Records of t3
-- ----------------------------
INSERT INTO `t3` VALUES ('1', 'downeyjr_1');
INSERT INTO `t3` VALUES ('2', 'downeyjr_2');
INSERT INTO `t3` VALUES ('3', 'downeyjr_3');
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