Mysql exists 與 in

今天公司同事反饋一個SQL語句刪除數據刪除了一個小時，尚未刪除完，強制中斷。 第一眼看到 exists 的時候，腦子裏要有這麼個概念：

Oracle exists 的效率比in 高。而Mysql 則不必定。 Mysql 使用eixsts 與使用in的規則爲：

子查詢的表大的時候，使用EXISTS能夠有效減小總的循環次數來提高速度；
外查詢的表大的時候，使用IN能夠有效減小對外查詢表循環遍從來提高速度。
從本質上講，exists 是之外查詢爲驅動表，而in 是以子查詢爲驅動表(驅動表決定了以 哪一個結果集做爲nestloop的對比依據)。

3.1.1 SQL

DELETE t FROM   o.`AI_AD_U_L` t   WHERE EXISTS (SELECT     1   FROM     o.`AI_AD_U_L_TEMP`  AS a   WHERE a.`ca_id`=t.`ca_id`);

3.1.2 分析過程

1. 查看錶上的索引

   mysql> show index from AI_AD_U_L;
   +-----------+------------+---------------------------------+--------------+--------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
   | Table     | Non_unique | Key_name                        | Seq_in_index | Column_name  | Collation | Cardinality | Sub_part | Packed | Null | Index_type | Comment | Index_comment |
   +-----------+------------+---------------------------------+--------------+--------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
   | AI_AD_U_L |          0 | PRIMARY                         |            1 | prod_inst_id | A         |    21162012 |     NULL | NULL   |      | BTREE      |         |               |
   | AI_AD_U_L |          1 | ai_sync_prod_level_cust_addr_id |            1 | cust_addr_id | A         |     8266746 |     NULL | NULL   | YES  | BTREE      |         |               |
   | AI_AD_U_L |          1 | ai_sync_prod_level_mac          |            1 | mac          | A         |    12227460 |     NULL | NULL   | YES  | BTREE      |         |               |
   +-----------+------------+---------------------------------+--------------+--------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
   3 rows in set (0.00 sec)
   mysql> show index from AI_AD_U_L_TEMP;
   +----------------+------------+-------------------+--------------+--------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
   | Table          | Non_unique | Key_name          | Seq_in_index | Column_name  | Collation | Cardinality | Sub_part | Packed | Null | Index_type | Comment | Index_comment |
   +----------------+------------+-------------------+--------------+--------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
   | AI_AD_U_L_TEMP |          1 | idx_cust_addr_id2 |            1 | cust_addr_id | A         |        2366 |     NULL | NULL   | YES  | BTREE      |         |               |
   | AI_AD_U_L_TEMP |          1 | idx_prod_inst_id  |            1 | prod_inst_id | A         |        3791 |     NULL | NULL   |      | BTREE      |         |               |
   +----------------+------------+-------------------+--------------+--------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
   2 rows in set (0.00 sec)

   此時表上是有對應字段的索引的，若是索引不存在，須要建立索引。

2. 查看執行計劃

   mysql> explain DELETE   t FROM   o.`AI_AD_U_L` t WHERE EXISTS   (SELECT     1   FROM     o.`AI_AD_U_L_TEMP` AS a   WHERE a.prod_inst_id = t.prod_inst_id);
   +----+--------------------+-------+------------+------+------------------+------------------+---------+-----------------------+----------+----------+-------------+
   | id | select_type        | table | partitions | type | possible_keys    | key              | key_len | ref                   | rows     | filtered | Extra       |
   +----+--------------------+-------+------------+------+------------------+------------------+---------+-----------------------+----------+----------+-------------+
   |  1 | DELETE             | t     | NULL       | ALL  | NULL             | NULL             | NULL    | NULL                  | 21162122 |   100.00 | Using where |
   |  2 | DEPENDENT SUBQUERY | a     | NULL       | ref  | idx_prod_inst_id | idx_prod_inst_id | 8       | o.t.prod_inst_id      |        1 |   100.00 | Using index |
   +----+--------------------+-------+------------+------+------------------+------------------+---------+-----------------------+----------+----------+-------------+
   2 rows in set, 1 warning (0.01 sec)

   經過執行計劃發現兩點問題：

   1. 外查詢表數據量大,21162122,也就是訪問了21162122次，而子查詢經過索引只訪問了一次。
   2. 發現子查詢使用了索引，而外查詢表上沒有使用索引。

   從以上兩點發現，說明外查詢做爲了驅動表。

3. 查看子查詢中表的數據量

   mysql> select count(*) from AI_AD_U_L_TEMP;
   +----------+
   | count(*) |
   +----------+
   |     3791 |
   +----------+
   1 row in set (0.00 sec)

   子查詢中數據量小，應以子查詢爲驅動表。應該用exists 應換成in。

4. 調整SQL語句並查看執行計劃 將exists 改成in 的用法 。

   mysql> explain DELETE   t FROM   o.`AI_AD_U_L` t WHERE t.prod_inst_id in  (SELECT prod_inst_id FROM     o.`AI_AD_U_L_TEMP` AS a   );
   +----+-------------+-------+------------+--------+------------------+------------------+---------+-----------------------+------+----------+------------------------+
   | id | select_type | table | partitions | type   | possible_keys    | key              | key_len | ref                   | rows | filtered | Extra                  |
   +----+-------------+-------+------------+--------+------------------+------------------+---------+-----------------------+------+----------+------------------------+
   |  1 | SIMPLE      | a     | NULL       | index  | idx_prod_inst_id | idx_prod_inst_id | 8       | NULL                  | 3791 |   100.00 | Using index; LooseScan |
   |  1 | DELETE      | t     | NULL       | eq_ref | PRIMARY          | PRIMARY          | 8       | o.a.prod_inst_id |    1 |   100.00 | NULL                   |
   +----+-------------+-------+------------+--------+------------------+------------------+---------+-----------------------+------+----------+------------------------+
   2 rows in set (0.00 sec)

   從執行計劃中能夠看到,兩張表都在使用索引。而外表的訪問次數也明顯降低爲子查詢表中的行數。大量減小了循環訪問外表的次數。

5. 執行SQL語句

   mysql> DELETE   t FROM   o.`AI_AD_U_L` t WHERE t.prod_inst_id in  (SELECT prod_inst_id FROM     o.`AI_AD_U_L_TEMP` AS a   );
   Query OK, 3525 rows affected (0.44 sec)

   咱們看到效果明顯, 原來1小時都沒法執行完成的SQL，如今只須要0.44秒。