MYSQL 中 exists 语句执行效率变低

在ORACLE 中,我们常常推荐使用exists 来替代in,往往也能取得比较好的优化效果。在ORACLE应用迁往MYSQL的过程中，我们发现部分in 的子查询语句带到MYSQL中，其执行效率变得非常低下，这很让人觉得匪夷所思。于是，我分析了一波。

对两个表，分别是一大一小进行关联查询:

mysql> select count(*) from users;+----------+| count(*) |+----------+|       19 |+----------+1 row in setmysql> select count(*) from orders;+----------+| count(*) |+----------+|    86310 |+----------+1 row in setmysql>

开启profile，发现无论是子查询是大表还是小表 ，exists的语句总是比in执行慢：

mysql> show profiles;+----------+------------+--------------------------------------------------------------------------------------------+| Query_ID | Duration   | Query                                                                                      |+----------+------------+--------------------------------------------------------------------------------------------+|        1 | 1.08661625 | select count(1) from orders o where o.user_id in(select u.id from users u)                 ||        2 | 1.56956275 | select count(1) from orders o where exists (select 1 from users u where u.id = o.user_id)  ||        3 | 0.81266425 | select count(1) from users u where u.id in(select o.user_id from orders o)                 ||        4 |  8.4164905 | select count(1) from users u where exists (select 1 from  orders o where u.id = o.user_id) |+----------+------------+--------------------------------------------------------------------------------------------+4 rows in set

而查看exists语句的profile内容，发现其存在多个executing 和sending data过程，这是整个sql执行的主要耗时过程：

mysql> show profile for query 2;+----------------------------+----------+| Status                     | Duration |+----------------------------+----------+| executing                  | 2E-6     || Sending data               | 1.2E-5   || executing                  | 2E-6     || Sending data               | 1.2E-5   || executing                  | 2E-6     || Sending data               | 1.2E-5   || executing                  | 2E-6     || Sending data               | 1.5E-5   || executing                  | 2E-6     || Sending data               | 1.2E-5   || executing                  | 2E-6     || Sending data               | 1.2E-5   || executing                  | 2E-6     || Sending data               | 1.2E-5   || executing                  | 2E-6     || Sending data               | 1.2E-5   || executing                  | 2E-6     || Sending data               | 1.2E-5   || executing                  | 2E-6     || Sending data               | 1.2E-5   || executing                  | 2E-6     || Sending data               | 1.2E-5   || executing                  | 2E-6     || Sending data               | 1.2E-5   || executing                  | 2E-6     || Sending data               | 1.2E-5   || executing                  | 2E-6     || Sending data               | 1.2E-5   || executing                  | 2E-6     || Sending data               | 1.2E-5   || executing                  | 2E-6     || Sending data               | 1.2E-5   || executing                  | 2E-6     || Sending data               | 1.2E-5   || executing                  | 2E-6     || Sending data               | 1.2E-5   || executing                  | 2E-6     || Sending data               | 1.2E-5   || executing                  | 1E-6     || Sending data               | 1.3E-5   || executing                  | 2E-6     || Sending data               | 1.2E-5   || executing                  | 2E-6     || Sending data               | 1.2E-5   || executing                  | 2E-6     || Sending data               | 1.2E-5   || executing                  | 2E-6     || Sending data               | 1.7E-5   || executing                  | 2E-6     || Sending data               | 1.2E-5   || executing                  | 2E-6     || Sending data               | 1.2E-5   || executing                  | 2E-6     || Sending data               | 1.2E-5   || executing                  | 2E-6     || Sending data               | 1.2E-5   || executing                  | 2E-6     || Sending data               | 1.2E-5   || executing                  | 2E-6     || Sending data               | 1.2E-5   || executing                  | 2E-6     || Sending data               | 1.2E-5   || executing                  | 2E-6     || Sending data               | 1.2E-5   || executing                  | 2E-6     || Sending data               | 1.1E-5   || executing                  | 2E-6     || Sending data               | 1.2E-5   || executing                  | 1E-6     || Sending data               | 1.2E-5   || executing                  | 2E-6     || Sending data               | 1.2E-5   || executing                  | 2E-6     || Sending data               | 1.2E-5   || executing                  | 2E-6     || Sending data               | 1.2E-5   || executing                  | 2E-6     || Sending data               | 1.2E-5   || executing                  | 2E-6     || Sending data               | 1.2E-5   || executing                  | 2E-6     || Sending data               | 1.2E-5   || executing                  | 2E-6     || Sending data               | 1.1E-5   || executing                  | 2E-6     || Sending data               | 1.2E-5   || executing                  | 2E-6     || Sending data               | 1.2E-5   || executing                  | 2E-6     || Sending data               | 1.2E-5   || executing                  | 2E-6     || Sending data               | 1.2E-5   || executing                  | 2E-6     || Sending data               | 1.5E-5   || end                        | 3E-6     || query end                  | 3E-6     || waiting for handler commit | 1E-5     || closing tables             | 9E-6     || freeing items              | 0.000152 || cleaning up                | 1.7E-5   |+----------------------------+----------+100 rows in set

而在in 子查询中，sending data这个过程只有一次，这也是整个sql执行主要耗时地方：

mysql> show profile for query 1;+--------------------------------+----------+| Status                         | Duration |+--------------------------------+----------+| starting                       | 9.3E-5   || Executing hook on transaction  | 6E-6     || starting                       | 8E-6     || checking permissions           | 5E-6     || checking permissions           | 4E-6     || Opening tables                 | 0.004849 || init                           | 1.8E-5   || System lock                    | 1.4E-5   || optimizing                     | 1.4E-5   || statistics                     | 3.1E-5   || preparing                      | 2.2E-5   || executing                      | 3E-6     || Sending data                   | 1.081273 | 《《《《《《《《《《《《《《《《《《《| end                            | 1.3E-5   || query end                      | 3E-6     || waiting for handler commit     | 1E-5     || closing tables                 | 5.2E-5   || freeing items                  | 0.000171 || cleaning up                    | 2.9E-5   |+--------------------------------+----------+

关于sending data和executing 解析：

Sending data

The thread is reading and processing rows for a SELECT statement, and sending data to the client. Because operations occurring during this state tend to perform large amounts of disk access (reads), it is often the longest-running state over the lifetime of a given query.

说Sending data 是：线程正在为一个select语句读取和处理行，并且发送数据到客户端。因为这期间操作倾向于大量的磁盘访问（读取），所以这常是整个查询周期中运行时间最长的阶段。（这是MySQL 5.5的解释，5.7的解释完全一样，但是5.7多了一个Sending to client状态）

这样就清楚了，Sending data 做了 读取，处理（过滤，排序等。。）和发送 三件事情，接下来再看该状态下的cpu 和 io 信息 以分析语句的瓶颈是 读取还是处理 ，再做相应的优化调整。。

executing

The thread has begun executing a statement.

在exists语句中不断地executing和Sending data 应该是在不断地扫描抓取数据进行匹配，那这应该与MYSQL 的算法有关，通过trace查看其实是在多次子查询 join_execution：

  {            "subselect_execution": {              "select#": 2,              "steps": [                {                  "join_execution": {                    "select#": 2,                    "steps": [                    ] /* steps */                  } /* join_execution */                }              ] /* steps */            } /* subselect_execution */          },

想知道这是鸡肋吗，不知有没有相关的解析？