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 0:4:48 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。