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

  • From: "Jonathan Lewis" <jonathan@xxxxxxxxxxxxxxxxxx>
  • To: <mgogala@xxxxxxxxxxxxxxxxxxxx>, <cary.millsap@xxxxxxxxxx>, "'Oracle-L (E-mail)'" <oracle-l@xxxxxxxxxxxxx>
  • Date: Fri, 14 Jan 2005 22:00:32 -0000

Note in-line


Jonathan Lewis
The Co-operative Oracle Users' FAQ
Public Appearances - schedule updated Dec 23rd 2004

Cary Millsap wrote:

>If I recall correctly, 10g has some operational memory built into the =
>that allows the CBO to use operationally measured response times for a =
>query as an input into computing the next plan.

I think you may be remembering the explanation I gave
in one of my Oracle 11! demonstrations to explain how
the HAL optimizer could change its mind in mid-stream.

When 9i came up with v$segstat and v$sql_plan_statistics,
it became obvious that Oracle Corp. could start writing
an extension to the optimizer that could use results from
previous queries, and object-level cache hit rates as the 
basis for changing its estimated cost of a query.


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