RE: CBO - default no of rows

From: Thomas, Kevin <Kevin.Thomas_at_calanais.com>
Date: Fri, 14 Sep 2001 03:10:13 -0700
Message-ID: <F001.0038E489.20010914031038@fatcity.com>

!! Please do not post Off Topic to this List !!

Ooooh, correct me if I'm wrong on this one, but doesn't the CBO just use the stats that are on the table. Hence if you actually delete the stats for the tables that don't have upto date stat values, you can actually get a performance increase,

my tuppence worth.

K.
"hit any user to continue"

Kevin Thomas
Technical Analyst
Deregulation Services
Calanais Ltd.
(2nd Floor East - Weirs Building)
Tel: 0141 568 2377
Fax: 0141 568 2366
http://www.calanais.com <http://www.calanais.com/>

-----Original Message-----
Sent: 14 September 2001 11:25
To: Multiple recipients of list ORACLE-L

My question of the day is :-
What value does the CBO use as a default number of rows for a table.

Background: -

We all know that if any tables in a query have been analyzed then CBO is used for the query not RBO (couple of caveats I know but let's continue).

So if 3 tables are used in a query and table a has 500 rows (analyzed) table b has 50 rows (never analyzed) and table c has 3500000 rows (never analyzed) all things being equal then CBO is used but what values does the CBO use for tables b or c to decide which execution plan is best.

On a development system yesterday a query was running slow. I realised that we had put in a very large data load (3.5M rows). I analyzed the table and indexes and the query came back in sub second response time. I am trying to figure what value was being used for num_rows prior to the analyze. It cannot be based on blocks allocated/used as thet would have increased after the dataload

Thanks

John

Oracle DBA
BTcellnet
* john.hallas_at_btcellnet.net

0113 388 6062 Desk
07713 066194 BT Mobile

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Author: Thomas, Kevin
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Received on Fri Sep 14 2001 - 05:10:13 CDT