Re: Creating Histograms
- From: "Jonathan Lewis" <jonathan@xxxxxxxxxxxxxxxxxx>
- To: <oracle-l@xxxxxxxxxxxxx>
- Date: Fri, 23 Jul 2004 17:50:29 +0100
Note in-line
Regards
Jonathan Lewis
http://www.jlcomp.demon.co.uk/faq/ind_faq.html
The Co-operative Oracle Users' FAQ
http://www.jlcomp.demon.co.uk/seminar.html
Optimising Oracle Seminar - schedule updated July 20th
----- Original Message -----
From: "Cary Millsap" <cary.millsap@xxxxxxxxxx>
>> Jonathan, would you agree that it's fair to say that, "Cost is irrelevant
>> for virtually every purpose other than comparing it with other costs?"
No. Actually I tend to use the cost to identify
a) problems with statistics
b) deficiences in the CBO
c) bugs in CBO
in that order.
Apart from that, I tend to assume that "cost = predicted
run time", with a fudge factor thrown in for known deficiencies
in the CBO. (I used to say cost = csecs to completion as a
rough guide, but that's just an approximation to the now-published
statement that the cost is the completion time measured in units of
the single-block read-time)
In fact, when cost != run-time, that's my first clue to the
presence of a CBO problem.
>> I know that it may be possible to find some factor f for which (response
>> time) = f * cost, but I think you and Tom would both say that cost is
>> predominantly of value as a measure that allows CBO to RANK the expected
>> performance of competing query execution plans. Yes?
No. The most significant difference of opinion that we have (or perhaps had)
was that Tom used to state quite firmly that you could not compare the
cost of one query with the cost of a different query and assume that the
comparison had any meaning. I believe that two costs reported at the
same time from the same system mean the same thing - even if they come
from the execution paths from two different queries.
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