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 0:4:48    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。