mysqlreport显示Com_中change_db占用比例高的问题的解决方法

最近公司的mysql服务器经常出现阻塞状态。动不动就重启，给用户访问带来了相当的不便。

首先说明一点，每天的访问量并不大每天才2W的访问量，按道理说，访问量再增加一倍这样的服务器也应该足以承受。可是我们的服务器为什么总是这样频频重启呢？经过mysqlreport显示mysql运行的状态如下：

MySQL 5.1.28-rc-log   uptime 0 0:4:48    Thu Apr 30 14:04:58 2009

__ Key _________________________________________________________________
Buffer used   7.74M of 384.00M %Used:  2.02
 Current   52.89M      %Usage: 13.77
Write hit    0.00%
Read hit    91.97%

__ Questions ___________________________________________________________
Total     14.46k  50.2/s
 DMS      9.24k  32.1/s %Total: 63.90
 QC Hits    3.66k  12.7/s      25.33
 Com_     1.04k   3.6/s      7.19
 COM_QUIT    517   1.8/s      3.58
 -Unknown     1   0.0/s      0.01
Slow 1 s      4   0.0/s      0.03 %DMS:  0.04 Log: OFF
DMS       9.24k  32.1/s      63.90
 SELECT    9.21k  32.0/s      63.75     99.75
 UPDATE      23   0.1/s      0.16     0.25
 REPLACE      0    0/s      0.00     0.00
 DELETE      0    0/s      0.00     0.00
 INSERT      0    0/s      0.00     0.00
Com_      1.04k   50/s      44.4
 set_option   523   1.8/s      3.62
 change_db    510   50/s      44.4
 show_proces    5   0.0/s      0.03

__ SELECT and Sort _____________________________________________________
Scan        49   0.2/s %SELECT:  0.53
Range       16   0.1/s      0.17
Full join      2   0.0/s      0.02
Range check     0    0/s      0.00
Full rng join    0    0/s      0.00
Sort scan     55   0.2/s
Sort range    475   1.6/s
Sort mrg pass    0    0/s

__ Query Cache _________________________________________________________
Memory usage  12.02M of 150.00M %Used:  8.01
Block Fragmnt  0.01%
Hits      3.66k  12.7/s
Inserts     9.14k  31.8/s
Insrt:Prune  9.14k:1  31.8/s
Hit:Insert   0.40:1

__ Table Locks _________________________________________________________
Waited       0    0/s %Total:  0.00
Immediate   10.49k  36.4/s

__ Tables ______________________________________________________________
Open        58 of 512  %Cache: 11.33
Opened       64   0.2/s

__ Connections _________________________________________________________
Max used      4 of 2000   %Max:  0.20
Total       519   1.8/s

__ Created Temp ________________________________________________________
Disk table     1   0.0/s
Table       184   0.6/s  Size: 256.0M
File        5   0.0/s

__ Threads _____________________________________________________________
Running       2 of  2
Cached       2 of 150   %Hit: 99.23
Created       4   0.0/s
Slow        0    0/s

__ Aborted _____________________________________________________________
Clients       0    0/s
Connects      0    0/s

__ Bytes _______________________________________________________________
Sent      20.36M  70.7k/s
Received    1.37M  4.7k/s

__ InnoDB Buffer Pool __________________________________________________
Usage     304.00k of  8.00M %Used:  3.71
Read hit    84.42%
Pages
 Free      493      %Total: 96.29
 Data       19           3.71 %Drty:  0.00
 Misc       0           0.00
 Latched                 0.00
Reads       77   0.3/s
 From file    12   0.0/s      15.58
 Ahead Rnd     1   0.0/s
 Ahead Sql     0    0/s
Writes       0    0/s
Flushes       0    0/s
Wait Free      0    0/s

__ InnoDB Lock _________________________________________________________
Waits        0    0/s
Current       0
Time acquiring
 Total       0 ms
 Average      0 ms
 Max        0 ms

__ InnoDB Data, Pages, Rows ____________________________________________
Data
 Reads      25   0.1/s
 Writes      3   0.0/s
 fsync       3   0.0/s
 Pending
  Reads      0
  Writes     0
  fsync      0

Pages
 Created      0    0/s
 Read       19   0.1/s
 Written      0    0/s

Rows
 Deleted      0    0/s
 Inserted     0    0/s
 Read       0    0/s
 Updated      0    0/s
大家可以看到在Com_中占的比例是相当大的，而一般情况下，这个值应该是3.0%左右。如果这个值过高的话，就说明，你的mysql处理请求中
做了一些无谓的工作，占用了大量的系统资源。其中我们看到尤为change_db的占用率比较高。这个值高了，说明我们执行的user database;命令相当多。
检查程序，原来我们在执行查询语句的时候应用了mysql_query_db("database","sql");这个语句在每次执行的时候都会自动调用use database;
所以我们应该将mysql_db_query 更改成mysql_query();再观察mysql的运行状态，ok。一切正常了。