Re: Problem with understanding Optimization methods.

From: Jonathan Lewis <jonathan_at_jlcomp.demon.co.uk>
Date: Thu, 08 Jan 2004 05:14:34 -0800
Message-ID: <F001.005DC257.20040108051434@fatcity.com>

Conversely, the CBO is a lot smarter with this scenario that people realise. How many people knew that Oracle can resolve a query of the type:

where colX is null
using a b-tree index ?

Try this --

drop table t1;

create table t1 (n1 number, n2 number not null, n3 number); create index i1 on t1 (n1, n2);

set autotrace traceonly explain
select /*+ first_rows */ * from t1 where n1 is null; set autotrace off

Execution Plan

0 SELECT STATEMENT Optimizer=HINT: FIRST_ROWS (Cost=4 Card=4 Bytes=156)

1 0 TABLE ACCESS (BY INDEX ROWID) OF 'T1' (TABLE) (Cost=4 Card=4 Bytes=156)

2 1 INDEX (RANGE SCAN) OF 'I1' (INDEX) (Cost=4 Card=4)

Regards

Jonathan Lewis
http://www.jlcomp.demon.co.uk

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Original Message ----- To: "Multiple recipients of list ORACLE-L" <ORACLE-L_at_fatcity.com> Sent: Thursday, January 08, 2004 11:54 AM

Bambi,

Yes it is expected behaviour, but only when it is guaranteed that no rows will be missed because of unindexed null entries. I wanted to point out that RBO is too "dumb" to realize that even though it ordered by column A which could be null, the column B in composite index was not null, thus causing every row to be indexed and RBO didn't use the index.

Tanel.

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Author: Jonathan Lewis
INET: jonathan_at_jlcomp.demon.co.uk

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