那天因参加MS的新品发布大会，中午就离开办公室，当我正在出租车上前往会场的途中，同事打电话来说主数据库出现写保护错误。
这可不得了，所有的应用都靠这个数据库呀，我心里默念：千万不要出漏子，否则就不能参会了！于是我在电话里交代同事重启mysql数据库试试，还好，重启后问题解决。
一散会，就赶紧上去找故障原因。这里先描述一下平台环境，把逻辑关系弄清楚。在这个应用中，由一个web前段服务器，一个tomcat应用服务器及一个mysql服务器构成，所有的系统都是linux。用户的请求先到前端的apache服务器，如果请求页面是.jsp，apache就把请求转交给tomcat服务器，tomcat再从数据库获取数据或向数据库插入记录。这是典型的3层应用逻辑。
登录到数据库mysql服务器，用mysql客户端连接mysql数据库，执行命令 mysql > show processlist; 没发现什么异常，负载也很低。看来从这里看不出什么名堂。接下来当然该看mysql错误日志，发现如下异常：
080313 11:25:35 InnoDB: Error: cannot allocate 1064960 bytes of
InnoDB: memory with malloc! Total allocated memory
InnoDB: by InnoDB 1233305429 bytes. Operating system errno: 12
InnoDB: Check if you should increase the swap file or
InnoDB: ulimits of your operating system.
InnoDB: On FreeBSD check you have compiled the OS with
InnoDB: a big enough maximum process size.
InnoDB: Note that in most 32-bit computers the process
InnoDB: memory space is limited to 2 GB or 4 GB.
InnoDB: We keep retrying the allocation for 60 seconds...
080313 11:26:08 [ERROR] /usr/local/mysql/bin/mysqld: Sort aborted
080313 11:26:19 InnoDB: Error: cannot allocate 1064960 bytes of
InnoDB: memory with malloc! Total allocated memory
这个报错的大意是：内存基本耗尽，没有再可以分配的空间。由此判断是什么东西产生巨大的负荷导致系统内存被榨干了。不过现在数据库服务器已经趋于平稳，暂时查不到什么原因引起这个故障。
基本情况掌握了，停下来休息片刻，于是顺手收一下邮件，乖乖，来了一封报警邮件，赶紧打开，其内容如下：
***** Nagios 2.9 *****
Notification Type: PROBLEM
Service: check_load
Host: tomcat nch100
Address: 61.154.105.100
State: WARNING
Date/Time: Thu Mar 13 10:59:53 CST 2008
Additional Info:
WARNING - load average: 3.94, 8.56, 9.17
报警消息表明主机61.154.105.100在10:59的这个时间负载过大。而这个主机正好是tomcat服务器,看来问题就在这个上面.为了近一步确认自己的想法,我来查看一下网络流量
从流量图可以看出,产生异常流量的时间正好与报警信息的时间一致,再给同事打电话,问：“你们都在61.154.105.100 这个机器上干了啥?”，答：“执行了一条不正确的sql语句，发现问题后取消这个sql语句”。至此，原因查明！
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What is the Using temporary status in EXPLAIN and how to avoid it?Apr 15, 2025 am 12:14 AMUsingtemporary indicates that the need to create temporary tables in MySQL queries, which are commonly found in ORDERBY using DISTINCT, GROUPBY, or non-indexed columns. You can avoid the occurrence of indexes and rewrite queries and improve query performance. Specifically, when Usingtemporary appears in EXPLAIN output, it means that MySQL needs to create temporary tables to handle queries. This usually occurs when: 1) deduplication or grouping when using DISTINCT or GROUPBY; 2) sort when ORDERBY contains non-index columns; 3) use complex subquery or join operations. Optimization methods include: 1) ORDERBY and GROUPB
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How does MySQL index cardinality affect query performance?Apr 14, 2025 am 12:18 AMMySQL index cardinality has a significant impact on query performance: 1. High cardinality index can more effectively narrow the data range and improve query efficiency; 2. Low cardinality index may lead to full table scanning and reduce query performance; 3. In joint index, high cardinality sequences should be placed in front to optimize query.
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Real-World MySQL: Examples and Use CasesApr 14, 2025 am 12:15 AMMySQL's real-world applications include basic database design and complex query optimization. 1) Basic usage: used to store and manage user data, such as inserting, querying, updating and deleting user information. 2) Advanced usage: Handle complex business logic, such as order and inventory management of e-commerce platforms. 3) Performance optimization: Improve performance by rationally using indexes, partition tables and query caches.
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