MySQL 为什么我只查一行的语句，也执行这么慢？
2019-11-13sad creeper
- MySQL 数据库本身就有很大的压力，导致数据库服务器 CPU 占用率很高或 ioutil（IO 利用率）很高，这种情况下所有语句的执行都有可能变慢。
- 等 MDL 锁。使用 show processlist 命令查看 Waiting for table metadata lock，出现这个状态表示的是，现在有一个线程正在表 x 上请求或者持有 MDL 写锁，把 select 语句堵住了。这类问题的处理方式，就是找到谁持有 MDL 写锁，然后把它 kill 掉，通过查询 sys.schema_table_lock_waits 这张表 mysql> SELECT blocking_pid FROM sys.schema_table_lock_waits。kill id
- 等 flush。表 x 上，执行下面的 SQL 语句：mysql> SELECT * FROM information_schema.processlist where id=1,出现 Waiting for table flush 状态的可能情况是：有一个 flush tables 命令被别的语句堵住了，然后它又堵住了我们的 select 语句。
- 等行锁。mysql> select * from x sys.innodb_lock_waits where locked_table=`'test'.'x'`\G, KILL id
- 查询慢。mysql> select * from t where c=50000 limit 1;由于字段 c 上没有索引，这个语句只能走 id 主键顺序扫描，因此需要扫描 5 万行
这其中涉及到了表锁、行锁和一致性读的概念
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