select * from searchzh where modified_date > '2009-09-02 14:45:22';
一条mysql查询语句的性能：sending data 耗时10分钟。。。。
+--------------------------------+-----------+
| Status | Duration |
+--------------------------------+-----------+
| (initialization) | 0.000006 |
| checking query cache for query | 0.000025 |
| checking permissions | 0.000008 |
| Opening tables | 0.000148 |
| System lock | 0.000008 |
| Table lock | 0.000008 |
| init | 0.000007 |
| checking permissions | 0.000045 |
| optimizing | 0.000016 |
| statistics | 0.00027 |
| preparing | 0.000034 |
| executing | 0.000005 |
| Sending data | 614.25387 |
| end | 0.000046 |
| query end | 0.000012 |
| freeing items | 0.000017 |
| closing tables | 0.000008 |
| logging slow query | 0.000054 |
+--------------------------------+-----------+
18 rows in set (0.01 sec)
产生的原因：
searchzh视图相关的表，update，insert，delete操作频繁。在mysql的表锁定机制中，select语句的优先级比update低。所以此select语句一直在锁队列中等待，后来的update操作会插队到select语句前面。导致| Sending data | 614.25387 |
解决方法有多种，最简单的：
select HIGH_PRIORITY * from searchzh where modified_date > '2009-09-02 14:45:22';
指定该select语句具有高优先级。
=======================================
mysql15:21:30> SHOW STATUS LIKE 'Table%';
+-----------------------+-----------+
| Variable_name | Value |
+-----------------------+-----------+
| Table_locks_immediate | 120977304 |
| Table_locks_waited | 577 |
+-----------------------+-----------+
表锁争用现象比较严重。
在频繁更新和查询的情景下，使用innodb也许更好。
转载
本文转自 geekwolf 51CTO博客，原文链接:http://blog.51cto.com/linuxgeek/995243
