（原创）
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
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 文档上给出的概念
以前经常在AWRRPT中看到CPU TIME 排在TOP5 EVENT的第一位，一直都不太清楚准确意思。这里给出了，CPU TIME就是实际执行或者等待操作系统运行队列所需要的时间，这里操作系统运行队列应该就是等待CPU时间片队列吧。但是还不准确，大概就可以理解是做实际操作需要的时间（比如进行SQL分析，执行，取回）。。
然后说了优化的目的，就是减少等待时间，比如减少ENQ锁等待，减少执行时间，比如减少逻辑读啊，游标共享啊。当然在OLTP和OLAP的优化不一样，OLAP一般瓶颈在大量的读，这个时候使用绑定变量视乎效果不好。
