Re: concatenated index

From: Mark Richard <mrichard_at_transurban.com.au>
Date: Mon, 07 Jul 2003 21:16:15 -0700
Message-ID: <F001.005C3BCF.20030707210925@fatcity.com>

I didn't mean that including RULE will prevent the index from being used - I should have explained myself better. When using the RULE based optimisor the order of joins in the where clause becomes significant, I don't believe the order of selection criteria is significant (although I have very little experience with the rule based optimisor and could easily be proven wrong).

If you really don't want Oracle to use the index (and assuming statistics are correct but you know something Oracle doesn't) then try:

/*+ NO_INDEX(test) */

Regards,

Mark.

                                                                                       
                                               
                      "Novice DBA"                                                     
                                               
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SQL> select /*+ RULE */
  2 A,B,C,D
  3 from test
  4 where b=113
  5 and c=114
  6 and a= 112
  7 /

A B C D

---------- ---------- ---------- ------------------------------
       112        113        114 test

Execution Plan

0 SELECT STATEMENT Optimizer=HINT: RULE 1 0 TABLE ACCESS (BY INDEX ROWID) OF 'TEST' 2 1 INDEX (RANGE SCAN) OF 'TEST_COMPOSITE' (NON-UNIQUE) Is there a problem in the way I have specified the hint. It still seems to be using the index.

Thanks in advance
Novice
No more Oracle certifiable DBA

>From: "Mark Richard" <[EMAIL PROTECTED]>
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>Subject: Re: concatenated index
>Date: Mon, 07 Jul 2003 20:09:24 -0800
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>
>The order of the where clause is not important. Including the leading
>(first) columns in the index is. If you remove the "a = ?" element from
>any of the queries then it may stop using the index. Oracle is smart
>enough to look at the entire where clause and work out what it can do to
>achieve the result quickest.
>
>You may be thinking on the rule based optimisor where the ordering of
where
>clauses is significant - for cost based optimisor the order is essentially
>irrelevant.
>
>Regards,
> Mark.
>
>
>
>
> "Novice DBA"
> <[EMAIL PROTECTED] To: Multiple
>recipients of list ORACLE-L <[EMAIL PROTECTED]>
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> Sent by: Subject: concatenated
>index
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> 08/07/2003 13:44
> Please respond to
> ORACLE-L
>
>
>
>
>
>
>Dear all,
> I have a basic doubt. I grew up (in Oracle) believing that for
the
>concatenated indexes to be used by a query the ordering of the columns in
>the where clause was very important. But now I have doubts
>I have a table test and a composite index on it.
>
>CREATE TABLE TEST (
> A NUMBER,
> B NUMBER,
> C NUMBER,
> D VARCHAR2 (30) ) ;
>
>
>CREATE INDEX TEST_COMPOSITE ON
> TEST(A, B, C)
>;
>
>I inserted some test data into it(376833 rows)
>
>Then tried some queries with explain plan. There is only one row which
>matches this criteria.
>
>SQL> explain plan for
> 2 select * from test
> 3 where a= 112
> 4 and b=113
> 5 and c=114;
>
>Explained.
>
>SQL> @plan
>
>Plan Table
>--------------------------------------------------------------------------------

>
>| SELECT STATEMENT | | 5K| 74K| 88 | |
>
> |
>| TABLE ACCESS BY INDEX ROW|TEST | 5K| 74K| 88 | |
>
> |
>| INDEX RANGE SCAN |TEST_COMP | 5K| | 39 | |
>
> |
>--------------------------------------------------------------------------------

>
>
>6 rows selected.
>
>SQL> explain plan for
> 2 select * from test
> 3 where a= 112
> 4 and c=114
> 5 and b=113;
>
>Explained.
>
>SQL> @plan
>
>Plan Table
>--------------------------------------------------------------------------------

>
>
>6 rows selected.
>
>SQL> explain plan for
> 2 select * from test
> 3 where b=113
> 4 and a= 112
> 5 and c=114;
>
>Explained.
>
>SQL> @plan
>
>Plan Table
>--------------------------------------------------------------------------------

>
>
>6 rows selected.
>
>SQL> explain plan for
> 2 select * from test
> 3 where b=113
> 4 and c=114
> 5 and a= 112;
>
>Explained.
>
>SQL> @plan
>
>Plan Table
>--------------------------------------------------------------------------------

>
>
>6 rows selected.
>
>SQL> explain plan for
> 2 select * from test
> 3 where b=113
> 4 and c=114;
>
>Explained.
>
>SQL> @plan
>
>Plan Table
>--------------------------------------------------------------------------------

>
>| SELECT STATEMENT | | 23K| 299K| 158 | |
>
> |
>| TABLE ACCESS FULL |TEST | 23K| 299K| 158 | |
>
> |
>--------------------------------------------------------------------------------

>
>
>
>Now I am at loss. I know this is something very basic. But I am unable to
>understand why the index is being used even when the order of the columns
>in
>the where clause is changed.
>
>Oracle version 8.1.7.2.
>
>
>Please enlighten me
>
>Thanks in advance
>
>Novice
>No more Oracle Certifiable DBA
>
>_________________________________________________________________
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Received on Mon Jul 07 2003 - 23:16:15 CDT