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

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

__ Key _________________________________________________________________
Buffer used   7.74M of 384.00M %Used:  2.02
 Current   52.89M  %Usage: 13.77
Write hit0.00%
Read hit91.97%

__ Questions ___________________________________________________________
Total 14.46k  50.2/s
 DMS  9.24k  32.1/s %Total: 63.90
 QC Hits3.66k  12.7/s  25.33
 Com_ 1.04k   3.6/s  7.19
 COM_QUIT517   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
 SELECT9.21k  32.0/s  63.75 99.75
 UPDATE  23   0.1/s  0.16 0.25
 REPLACE  00/s  0.00 0.00
 DELETE  00/s  0.00 0.00
 INSERT  00/s  0.00 0.00
Com_  1.04k   50/s  44.4
 set_option   523   1.8/s  3.62
 change_db510   50/s  44.4
 show_proces5   0.0/s  0.03

__ SELECT and Sort _____________________________________________________
Scan49   0.2/s %SELECT:  0.53
Range   16   0.1/s  0.17
Full join  2   0.0/s  0.02
Range check 00/s  0.00
Full rng join00/s  0.00
Sort scan 55   0.2/s
Sort range475   1.6/s
Sort mrg pass00/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   00/s %Total:  0.00
Immediate   10.49k  36.4/s

__ Tables ______________________________________________________________
Open58 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
File5   0.0/s

__ Threads _____________________________________________________________
Running   2 of  2
Cached   2 of 150   %Hit: 99.23
Created   4   0.0/s
Slow00/s

__ Aborted _____________________________________________________________
Clients   00/s
Connects  00/s

__ Bytes _______________________________________________________________
Sent  20.36M  70.7k/s
Received1.37M  4.7k/s

__ InnoDB Buffer Pool __________________________________________________
Usage 304.00k of  8.00M %Used:  3.71
Read hit84.42%
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 file12   0.0/s  15.58
 Ahead Rnd 1   0.0/s
 Ahead Sql 00/s
Writes   00/s
Flushes   00/s
Wait Free  00/s

__ InnoDB Lock _________________________________________________________
Waits00/s
Current   0
Time acquiring
 Total   0 ms
 Average  0 ms
 Max0 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  00/s
 Read   19   0.1/s
 Written  00/s

Rows
 Deleted  00/s
 Inserted 00/s
 Read   00/s
 Updated  00/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。