Re: Which plan is better - what COST really means ?

  • From: J.Velikanovs@xxxxxxxx
  • To: premj@xxxxxxxxxxxxxx
  • Date: Thu, 13 Jan 2005 12:46:16 +0200

Prem, I have question for you:
How would you decide which execution plan of the same query is better, 
when you are not allowed to run the query and to evaluate execution time?
CBO is in this situation. 
It has limited information regarding data structures (structure, 
statistics, binds etc.).
It?s trying to evaluate each plan by estimating its cost. 
Cost is the measure how bad/good plan is by CBO opinion. 
Cost has nothing to do with real execution time.


On 2005.01.13 10:57:11 oracle-l-bounce wrote:

>when i have got two plans , how do i decide
>which one is the best ?
>just read in asktom , that COST of a plan ,
>does not have any correspondence with response
>time of a query.
>i.e.,a query with higher COST than another
>can be faster.
>if so , what in the execution plan does help
>me to decide about a good (faster) plan ?
>unless i execute it , can't i know it ?
>is that the only way !!!
>Can someone throw light on this ?
>The environment is oracle
>Thanks in advance.

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