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"
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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
--------------------------------------------------------------------------------
| 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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