Question

我有两张桌子，其中一张桌子有一个小柱子。当我离开加入这张桌子时，我得到的结果非常快。但反过来说没有用。

 CREATE TABLE `userprofile_dtl` (
  `user_id` varchar(100) NOT NULL,
  `bankpassbook_doc` varchar(4) DEFAULT NULL,
  `aadhaar_no` tinyblob,
  PRIMARY KEY (`user_id`),
  KEY `idx_aadhaar_no_tinyblob` (`aadhaar_no`(16)) USING BTREE
) ENGINE=InnoDB DEFAULT CHARSET=utf8

 CREATE TABLE `commonformfields_dtl` (
  `app_id` varchar(25) NOT NULL,
  `user_id` varchar(100) DEFAULT NULL,
  `isParentsSeparated` varchar(3) DEFAULT NULL,
  PRIMARY KEY (`app_id`),
  KEY `user_id` (`user_id`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8

mysql如何使用tinyblob列上构建的完全不相关的密钥来比较user_id？
为什么它比使用primary的逻辑索引更快？

快速：

mysql>  explain 
SELECT count(*) 
FROM `userprofile_dtl` as a 
left join `commonformfields_dtl` as b 
  on a.user_id = b.user_id 
where b.user_id is null;

+----+-------------+-------+------------+-------+---------------+-------------------------+---------+---------------------+--------+----------+--------------------------+
| id | select_type | table | partitions | type  | possible_keys | key                     | key_len | ref                 | rows   | filtered | Extra                    |
+----+-------------+-------+------------+-------+---------------+-------------------------+---------+---------------------+--------+----------+--------------------------+
|  1 | SIMPLE      | a     | NULL       | index | NULL          | idx_aadhaar_no_tinyblob | 19      | NULL                | 392608 |   100.00 | Using index              |
|  1 | SIMPLE      | b     | NULL       | ref   | user_id       | user_id                 | 303     | portal.a.user_id |      1 |   100.00 | Using where; Using index |
+----+-------------+-------+------------+-------+---------------+-------------------------+---------+---------------------+--------+----------+--------------------------+

慢速：

mysql> explain   
SELECT count(*) 
FROM  `commonformfields_dtl` as a 
left join `userprofile_dtl` as b 
  on a.user_id = b.user_id 
where b.user_id is null;

+----+-------------+-------+------------+--------+---------------+---------+---------+---------------------+---------+----------+--------------------------------------+
| id | select_type | table | partitions | type   | possible_keys | key     | key_len | ref                 | rows    | filtered | Extra                                |
+----+-------------+-------+------------+--------+---------------+---------+---------+---------------------+---------+----------+--------------------------------------+
|  1 | SIMPLE      | a     | NULL       | index  | NULL          | user_id | 303     | NULL                | 2004994 |   100.00 | Using index                          |
|  1 | SIMPLE      | b     | NULL       | eq_ref | PRIMARY       | PRIMARY | 302     |  portal.a.user_id |       1 |   100.00 | Using where; Not exists; Using index |
+----+-------------+-------+------------+--------+---------------+---------+---------+---------------------+---------+----------+--------------------------------------+

mysql> explain   
SELECT count(*) 
FROM  `commonformfields_dtl` as a 
left join `userprofile_dtl` as b ignore index (primary) 
  on a.user_id = b.user_id 
where b.user_id is null;

+----+-------------+-------+------------+-------+---------------+-------------------------+---------+------+---------+----------+-----------------------------------------------------------------------------+
| id | select_type | table | partitions | type  | possible_keys | key                     | key_len | ref  | rows    | filtered | Extra                                                                       |
+----+-------------+-------+------------+-------+---------------+-------------------------+---------+------+---------+----------+-----------------------------------------------------------------------------+
|  1 | SIMPLE      | a     | NULL       | index | NULL          | user_id                 | 303     | NULL | 2004994 |   100.00 | Using index                                                                 |
|  1 | SIMPLE      | b     | NULL       | index | NULL          | idx_aadhaar_no_tinyblob | 19      | NULL |  392608 |    10.00 | Using where; Not exists; Using index; Using join buffer (Block Nested Loop) |
+----+-------------+-------+------------+-------+---------------+-------------------------+---------+------+---------+----------+-----------------------------------------------------------------------------+

找到一个表中但不存在于其他表中的记录数

0 个答案: