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

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 0:4:48Thu 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 14.46k50.2/s
 DMS9.24k32.1/s %Total: 63.90
 QC Hits3.66k12.7/s25.33
 Com_ 1.04k 3.6/s7.19
 COM_QUIT517 1.8/s3.58
 -Unknown 1 0.0/s0.01
Slow 1 s4 0.0/s0.03 %DMS:0.04 Log: OFF
DMS 9.24k32.1/s63.90
 SELECT9.21k32.0/s63.75 99.75
 UPDATE23 0.1/s0.16 0.25
 REPLACE00/s0.00 0.00
 DELETE00/s0.00 0.00
 INSERT00/s0.00 0.00
Com_1.04k 50/s44.4
 set_option 523 1.8/s3.62
 change_db510 50/s44.4
 show_proces5 0.0/s0.03

__ SELECT and Sort _____________________________________________________
Scan49 0.2/s %SELECT:0.53
Range 16 0.1/s0.17
Full join2 0.0/s0.02
Range check 00/s0.00
Full rng join00/s0.00
Sort scan 55 0.2/s
Sort range475 1.6/s
Sort mrg pass00/s

__ Query Cache _________________________________________________________
Memory usage12.02M of 150.00M %Used:8.01
Block Fragmnt0.01%
Hits3.66k12.7/s
Inserts 9.14k31.8/s
Insrt:Prune9.14k:131.8/s
Hit:Insert 0.40:1

__ Table Locks _________________________________________________________
Waited 00/s %Total:0.00
Immediate 10.49k36.4/s

__ Tables ______________________________________________________________
Open58 of 512%Cache: 11.33
Opened 64 0.2/s

__ Connections _________________________________________________________
Max used4 of 2000 %Max:0.20
Total 519 1.8/s

__ Created Temp ________________________________________________________
Disk table 1 0.0/s
Table 184 0.6/sSize: 256.0M
File5 0.0/s

__ Threads _____________________________________________________________
Running 2 of2
Cached 2 of 150 %Hit: 99.23
Created 4 0.0/s
Slow00/s

__ Aborted _____________________________________________________________
Clients 00/s
Connects00/s

__ Bytes _______________________________________________________________
Sent20.36M70.7k/s
Received1.37M4.7k/s

__ InnoDB Buffer Pool __________________________________________________
Usage 304.00k of8.00M %Used:3.71
Read hit84.42%
Pages
 Free493%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/s15.58
 Ahead Rnd 1 0.0/s
 Ahead Sql 00/s
Writes 00/s
Flushes 00/s
Wait Free00/s

__ InnoDB Lock _________________________________________________________
Waits00/s
Current 0
Time acquiring
 Total 0 ms
 Average0 ms
 Max0 ms

__ InnoDB Data, Pages, Rows ____________________________________________
Data
 Reads25 0.1/s
 Writes3 0.0/s
 fsync 3 0.0/s
 Pending
Reads0
Writes 0
fsync0

Pages
 Created00/s
 Read 19 0.1/s
 Written00/s

Rows
 Deleted00/s
 Inserted 00/s
 Read 00/s
 Updated00/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。