concatenated index

From: Novice DBA <novicedba_at_hotmail.com>
Date: Mon, 07 Jul 2003 19:51:12 -0700
Message-ID: <F001.005C3BC1.20030707194425@fatcity.com>

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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Author: Novice DBA
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