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
The educated person is not the person
who can answer the questions, but the
person who can question the answers -- T. Schick Jr
Next public appearance2:
March 2004 Hotsos Symposium - Keynote
March 2004 Charlotte NC - OUG Tutorial
April 2004 Iceland
One-day tutorials:
http://www.jlcomp.demon.co.uk/tutorial.html
Three-day seminar:
see http://www.jlcomp.demon.co.uk/seminar.html
____UK___February
The Co-operative Oracle Users' FAQ
http://www.jlcomp.demon.co.uk/faq/ind_faq.html
----- Original Message -----
To: "Multiple recipients of list ORACLE-L" <[EMAIL PROTECTED]>
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
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