RE: Max permutations
- From: John Kanagaraj <john.kanagaraj@xxxxxxx>
- To: "'oracle-l@xxxxxxxxxxxxx'" <oracle-l@xxxxxxxxxxxxx>
- Date: Mon, 9 Feb 2004 14:49:45 -0800
I just checked the autographed copy of OPT 101. Paraphrasing Gaja/Kirti's
words, when this is set to a value below 80,000, the optimizer is forced to
try upto eight different tables as the driving tables for queries that
involve joins.
OTOH, I was digging through some of my old email from another list (Oracle
Apps) where _very_ complex and large table joins are common-place, someone
reported that the time taken to execute (i.e. parse + fetch) was as below:
optimizer_max_permutations = 50 Query Time = 42 sec.
optimizer_max_permutations = 100 Query Time = 50 sec.
optimizer_max_permutations = 500 Query Time = 3 min.
optimizer_max_permutations = 1,000 Query Time = 6 min.
optimizer_max_permutations = 10,000 Query Time = 55 min.
optimizer_max_permutations = 50,000 Query Time = 117 min.
optimizer_max_permutations = 80,000 Query Time = 219 min.
Dunno why so many combinations were tested though! Nor do I have the SQL or
explain plans... :(
I do know however, that this was on 8.1.7.2 (i.e an erly version of 8.1.7)
and that Oracle recommends a setting of 2000 for OMP in Oracle Apps 11i
installs.
John Kanagaraj
DB Soft Inc
Phone: 408-970-7002 (W)
Listen to great, commercial-free christian music 24x7x365 at
http://www.klove.com
** The opinions and facts contained in this message are entirely mine and do
not reflect those of my employer or customers **
>-----Original Message-----
>From: oracle-l-bounce@xxxxxxxxxxxxx
>[mailto:oracle-l-bounce@xxxxxxxxxxxxx] On Behalf Of Wolfgang Breitling
>Sent: Monday, February 09, 2004 12:42 PM
>To: oracle-l@xxxxxxxxxxxxx
>Subject: Re: Max permutations
>
>
>Dave Ensor, in his presentation at UKOUG claims
>- reduce the setting (say to 40,320) and
> - Oracle uses an enhanced strategy to decide which join
>orders to evaluate
> - very significantly increases the probability of picking
>the correct
>driving table for a join of more than 8 tables
> - decreases parse time for joins of more than 8 tables
>
>Gaja, Kirti, et al. also claim in their book "Performance
>Tuning 101" that
>OMP 79,999 and 79,998 have "magic" powers. That was for Oracle 8.
>
>That last piece is from memory, so it may not be correct. The
>book is at
>home and I'm at a client's site.
>Kirti, care to confirm/deny/comment.
>
>At 08:51 AM 2/9/2004, you wrote:
>
>>Has anyone done any recent testing on the
>>effect of optimizer_max_permutations.
>>
>>I recall seeing a note on metalink once said
>>the CBO would change the way in which it
>>permuted join orders if the parameter was
>>set to any value other than 80,000. I'm also
>>fairly sure that I ran up a test a few years
>>ago that demonstrated this effect.
>>
>>However, I've just run up a simple test on
>>8.1.7.4 and 9.2.0.4 where the only change
>>was the number of join orders examined
>>before the optimizer stopped (a few hundred
>>for omp = 2000, a couple of thousand for
>>omp-80000) - the permutation sequences were
>>was identical.
>
>Wolfgang Breitling
>Oracle7, 8, 8i, 9i OCP DBA
>Centrex Consulting Corporation
>http://www.centrexcc.com
>
>
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