DEPENDENT SUBQUERY” 和 “SUBQUERY”

Posted 2020-07-28 zengkefu

http://blog.163.com/li_hx/blog/static/183991413201642410122327/

mysql> CREATE TABLE t1 (a INT, b INT);
Query OK, 0 rows affected (0.22 sec)

mysql> CREATE TABLE t2 (c INT, d INT);
Query OK, 0 rows affected (0.21 sec)

mysql> CREATE TABLE t3 (e INT);
Query OK, 0 rows affected (0.22 sec)

mysql> INSERT INTO t1 VALUES (1,10), (2,10);
Query OK, 2 rows affected (0.19 sec)
Records: 2  Duplicates: 0  Warnings: 0

mysql> INSERT INTO t2 VALUES (2,10), (2,20);
Query OK, 2 rows affected (0.17 sec)
Records: 2  Duplicates: 0  Warnings: 0

mysql> INSERT INTO t3 VALUES (10), (30);
Query OK, 2 rows affected (0.18 sec)
Records: 2  Duplicates: 0  Warnings: 0

mysql> select * from t1;
+------+------+
| a    | b    |
+------+------+
|    1 |   10 |
|    2 |   10 |
+------+------+
2 rows in set (0.00 sec)

mysql> select * from t2;
+------+------+
| c    | d    |
+------+------+
|    2 |   10 |
|    2 |   20 |
+------+------+
2 rows in set (0.00 sec)

mysql> select * from t3;
+------+
| e    |
+------+
|   10 |
|   30 |
+------+
2 rows in set (0.00 sec)

mysql> EXPLAIN SELECT * FROM t1 WHERE t1.a IN (SELECT c FROM t2 WHERE (SELECT e FROM t3 as t31) < SOME (SELECT e FROM t3 as t32 WHERE t1.b));
+----+--------------------+-------+------+---------------+------+---------+------+------+--------------------------------------------------------------------+
| id | select_type        | table | type | possible_keys | key  | key_len | ref  | rows | Extra                                                              |
+----+--------------------+-------+------+---------------+------+---------+------+------+--------------------------------------------------------------------+
|  1 | PRIMARY            | t1    | ALL  | NULL          | NULL | NULL    | NULL |    2 | Using where                                                        |
|  1 | PRIMARY            | t2    | ALL  | NULL          | NULL | NULL    | NULL |    2 | Using where; FirstMatch(t1); Using join buffer (Block Nested Loop) |
|  4 | DEPENDENT SUBQUERY | t32   | ALL  | NULL          | NULL | NULL    | NULL |    2 | Using where                                                        |
|  3 | SUBQUERY           | t31   | ALL  | NULL          | NULL | NULL    | NULL |    2 | NULL                                                               |
+----+--------------------+-------+------+---------------+------+---------+------+------+--------------------------------------------------------------------+
4 rows in set (0.00 sec)

mysql> EXPLAIN format=json SELECT * FROM t1 WHERE t1.a IN (SELECT c FROM t2 WHERE (SELECT e FROM t3 as t31) < SOME (SELECT e FROM t3 as t32 WHERE t1.b))\G
*************************** 1. row ***************************
EXPLAIN: {
  "query_block": {
    "select_id": 1,
    "nested_loop": [
      {
        "table": {
          "table_name": "t1",
          "access_type": "ALL",
          "rows": 2,
          "filtered": 100,
          "attached_condition": "<nop>(<in_optimizer>((/* select#3 */ select `test`.`t31`.`e` from `test`.`t3` `t31`),<exists>(/* select#4 */ select 1 from `test`.`t3` `t32` where (`test`.`t1`.`b` and <if>(outer_field_is_not_null, ((<cache>((/* select#3 */ select `test`.`t31`.`e` from `test`.`t3` `t31`)) < `test`.`t32`.`e`) or isnull(`test`.`t32`.`e`)), true)) having <if>(outer_field_is_not_null, <is_not_null_test>(`test`.`t32`.`e`), true))))",
          "attached_subqueries": [
            {
              "dependent": true,
              "cacheable": false,
              "query_block": {
                "select_id": 4,
                "table": {
                  "table_name": "t32",
                  "access_type": "ALL",
                  "rows": 2,
                  "filtered": 100,
                  "attached_condition": "(`test`.`t1`.`b` and <if>(outer_field_is_not_null, ((<cache>((/* select#3 */ select `test`.`t31`.`e` from `test`.`t3` `t31`)) < `test`.`t32`.`e`) or isnull(`test`.`t32`.`e`)), true))"
                }
              }
            },
            {
              "dependent": false,
              "cacheable": true,
              "query_block": {
                "select_id": 3,
                "table": {
                  "table_name": "t31",
                  "access_type": "ALL",
                  "rows": 2,
                  "filtered": 100
                }
              }
            }
          ]
        }
      },
      {
        "table": {
          "table_name": "t2",
          "access_type": "ALL",
          "rows": 2,
          "filtered": 100,
          "first_match": "t1",
          "using_join_buffer": "Block Nested Loop",
          "attached_condition": "(`test`.`t2`.`c` = `test`.`t1`.`a`)"
        }
      }
    ]
  }
}
1 row in set, 2 warnings (0.00 sec)

mysql> SHOW WARNINGS\G
*************************** 1. row ***************************
  Level: Note
   Code: 1276
Message: Field or reference ‘test.t1.b‘ of SELECT #4 was resolved in SELECT #1
*************************** 2. row ***************************
  Level: Note
   Code: 1003
Message: /* select#1 */ select `test`.`t1`.`a` AS `a`,`test`.`t1`.`b` AS `b` from `test`.`t1` semi join (`test`.`t2`) where ((`test`.`t2`.`c` = `test`.`t1`.`a`) and <nop>(<in_optimizer>((/* select#3 */ select `test`.`t31`.`e` from `test`.`t3` `t31`),<exists>(/* select#4 */ select 1 from `test`.`t3` `t32` where (`test`.`t1`.`b` and <if>(outer_field_is_not_null, ((<cache>((/* select#3 */ select `test`.`t31`.`e` from `test`.`t3` `t31`)) < `test`.`t32`.`e`) or isnull(`test`.`t32`.`e`)), true)) having <if>(outer_field_is_not_null, <is_not_null_test>(`test`.`t32`.`e`), true)))))
2 rows in set (0.00 sec)