Detailed actual combat: selectType analysis

1. Common selectType description

select_type query type	illustrate
SIMPLE	Simple select query without using union and subquery
PRIMARY	The outermost select query
SUBQUERY	A subquery is included in select or where, the first select query in the subquery does not depend on the result set of the outer query
DEPENDENT SUBQUERY	The first select query in a subquery that depends on the result set of the outer query
DERIVED	Used when there are subqueries in the from clause. MySQL will recursively execute these subqueries, placing the results in temporary tables.
UNION	A second or subsequent select query in a UNION that does not depend on the result set of the outer query
DEPENDENT UNION	A second or subsequent select query in a UNION that depends on the result set of the outer query
UNCACHEABLE UNION	The second or subsequent select query in UNION is a non-cacheable subquery

2. Examples of common selectType

• SIMPLE: simple select query, without using union and subquery
• PRIMARY: That is, the last executed statement
• SUBQUERY: contains subqueries in select or where
• DERIVED: Temporary tables will increase the burden on MYSQL, but sometimes they have to be used, analogy: two variables in Java exchange values
• UNION: The query results of two tables are merged

mysql> EXPLAIN SELECT t1.id from t1 UNION SELECT t2.id from t2;
+----+--------------+------------+------+---------------+------+---------+------+------+-------+
| id | select_type  | table      | type | possible_keys | key  | key_len | ref  | rows | Extra |
+----+--------------+------------+------+---------------+------+---------+------+------+-------+
|  1 | PRIMARY      | t1         | ALL  | NULL          | NULL | NULL    | NULL |    3 |       |
|  2 | UNION        | t2         | ALL  | NULL          | NULL | NULL    | NULL |    3 |       |
| NULL | UNION RESULT | <union1,2> | ALL  | NULL          | NULL | NULL    | NULL | NULL |       |
+----+--------------+------------+------+---------------+------+---------+------+------+-------+

• UNION RESULT: query again from the combined results

mysql> EXPLAIN SELECT t3.id from (SELECT t1.id from t1 UNION SELECT t2.id from t2)t3;
+----+--------------+------------+------+---------------+------+---------+------+------+-------+
| id | select_type  | table      | type | possible_keys | key  | key_len | ref  | rows | Extra |
+----+--------------+------------+------+---------------+------+---------+------+------+-------+
|  1 | PRIMARY      | <derived2> | ALL  | NULL          | NULL | NULL    | NULL |    3 |       |
|  2 | DERIVED      | t1         | ALL  | NULL          | NULL | NULL    | NULL |    3 |       |
|  3 | UNION        | t2         | ALL  | NULL          | NULL | NULL    | NULL |    3 |       |
| NULL | UNION RESULT | <union2,3> | ALL  | NULL          | NULL | NULL    | NULL | NULL |       |
+----+--------------+------------+------+---------------+------+---------+------+------+-------+

3. SelectType usage summary:

• selectType meaning: the type of select query, mainly to distinguish between ordinary queries and complex queries such as joint queries and subqueries.
• selectType function: identify the type of query and adjust the query method

4. Executed SQL file

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');