RE: Question on how cardinality is calculated.

All,  

Thanks to those who answered, the majority pointing out that it is 5% of the number of rows.  

The oracle version is 9.2.0.8  

To build on the information provided, the actual query used in an application does the following:  

select cases.a, cases.b, cases.c, cases.case, enquiry.a, enquiry.b, enquiry.c from cases, enquiry
where
enquiry.id = cases.id
and upper(case) like '%123456%';

Now the user always enters six digits which I know identifies 1 record in the cases table, but because Oracle uses a cardinality of 5553 for this it performs a hash join to the enquiry table, even though only one record will match in the enquiry table. The enquiry table contains 1.4 million record so it takes ~5 seconds.

Would it be advisable to put a hint on the query to either use a nested loop which allows the query to use an index to retrieve the data from enquiry table. Or use a cardinality hint since I know we will not be returning 5553 rows which will also allow the use of an index.  

Thanks

Ronnie

From: oracle-l-bounce_at_freelists.org on behalf of Jonathan Lewis Sent: Thu 1/10/2008 4:22 PM
To: Ronnie Doggart; oracle-l_at_freelists.org Subject: Re: Question on how cardinality is calculated.

5% of num_rows

Regards

Jonathan Lewis
http://jonathanlewis.wordpress.com <http://jonathanlewis.wordpress.com/>

Author: Cost Based Oracle: Fundamentals
http://www.jlcomp.demon.co.uk/cbo_book/ind_book.html

The Co-operative Oracle Users' FAQ
http://www.jlcomp.demon.co.uk/faq/ind_faq.html

• Original Message ----- From: "Ronnie Doggart" <ronnie_doggart_at_lagan.com> To: <oracle-l_at_freelists.org> Sent: Thursday, January 10, 2008 3:06 PM Subject: Question on how cardinality is calculated.

All

Does anyone know how Oracle calculates the cardinality of a query such as:

Select * from table_1 where upper(case) like '%12345%';

Execution Plan

   0 SELECT STATEMENT Optimizer=CHOOSE (Cost=131 Card=5553 Bytes=621936)    1 0 TABLE ACCESS (FULL) OF 'TABLE_1' (Cost=131 Card=5553 Bytes=621936)

Oracle calculates that it is expecting 5553 rows from this query but I know it will only return 2.

How does it do its estimate ?

Ronnie Doggart

--

http://www.freelists.org/webpage/oracle-l

The information in this message is confidential and may be legally privileged. It is intended solely for the addressee. Access to this message by anyone else is unauthorised. If you are not the intended recipient, any disclosure, copying, or distribution of the message, or any action or omission taken by you in reliance on it, is prohibited and may be unlawful. Please immediately contact the sender if you have received this message in error.

The views and opinions expressed in this email may not reflect the views and opinions of any member of Lagan Technologies Limited, or any of its subsidiaries. Lagan Technologies Limited is a company registered in Northern Ireland with registration number NI 28773. The registered office of Lagan Technologies Limited is 209 Airport Road West, Belfast, Co. Antrim, BT3 9EZ.

--

http://www.freelists.org/webpage/oracle-l Received on Thu Jan 10 2008 - 11:32:48 CST