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Re: concatenated index

From: Novice DBA <novicedba_at_hotmail.com>
Date: Mon, 07 Jul 2003 21:01:17 -0700
Message-ID: <F001.005C3BCC.20030707205424@fatcity.com> 





Hi



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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To: Multiple recipients of list ORACLE-L <[EMAIL PROTECTED]>
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]>
                      l.com>                   cc:
                      Sent by:                 Subject:  concatenated 
index
                      [EMAIL PROTECTED]
                      .com

                      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



| Operation                 |  Name    |  Rows | Bytes|  Cost  | Pstart|


Pstop |




| 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



| Operation                 |  Name    |  Rows | Bytes|  Cost  | Pstart|


Pstop |




| 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 b=113


  4  and a= 112


  5  and c=114;


Explained.



SQL> @plan



Plan Table



| Operation                 |  Name    |  Rows | Bytes|  Cost  | Pstart|


Pstop |




| 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 b=113


  4  and c=114


  5  and a= 112;


Explained.



SQL> @plan



Plan Table



| Operation                 |  Name    |  Rows | Bytes|  Cost  | Pstart|


Pstop |




| 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 b=113


  4  and c=114;


Explained.



SQL> @plan



Plan Table



| Operation                 |  Name    |  Rows | Bytes|  Cost  | Pstart|


Pstop |




| 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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