oracle中WAIT TIME 和 CPU TIME

（原創）
DB time =DB wait time +DB CPU time
DB Time
Tuning is not just about reducing waits. Its goal is to improve end-user response time and/or minimize the average resources used by each individual request. Sometimes these
go together, and in other cases there is a trade-off (for example, parallel query). In general,we can say that tuning is the avoidance of consuming or holding resources in a wasteful manner.
Any request to the database is composed of two distinct segments: a wait time (DB waittime) and a service time (DB CPU time). The wait time is the sum of all the waits for various resources. The CPU time is the sum of the time that is spent actually working on the request or waiting on the OS run queue. These times are not necessarily composed of one wait and one block of CPU time.
Tuning consists of reducing or eliminating the wait time and reducing the CPU time.
This definition applies to any application type: online transaction processing (OLTP) or
data warehouse (DW).
Note: A very busy system shows longer DB CPU time due to waits on the run queue.
An overloaded system causes processes

以上是ORACLE 文件上給出的概念

以前經常在AWRRPT中看到CPU TIME 排在TOP5 EVENT的第一位，一直都不太清楚準確意思。這裡給出了，CPU TIME就是實際執行或者等待作業系統執行佇列所需要的時間，這裡作業系統執行佇列應該就是等待CPU時間片佇列吧。但是還不準確，大概就可以理解是做實際操作需要的時間（比如進行SQL分析，執行，取回）。。

  然後說了最佳化的目的，就是減少等待時間，比如減少ENQ鎖等待，減少執行時間，比如減少邏輯讀啊，遊標共享啊。當然在OLTP和OLAP的最佳化不一樣，OLAP一般瓶頸在大量的讀，這個時候使用繫結變數視乎效果不好。