-->寻找低效的SQL语句 ,下面的语句主要适用于从视图v$sqlarea中获得当前运行下且耗用buffer_gets较多的SQL语句                   
SELECT executions                                                                     
     , disk_reads                                                                    
     , buffer_gets                                                                  
     , ROUND( ( buffer_gets         
               - disk_reads )       
             / buffer_gets, 2 )      
          hit_ratio                                      
     , ROUND( disk_reads / executions, 2 ) reads_per_run                   
     , sql_text                                       
FROM   v$sqlarea                                                               
WHERE      executions > 0    &n