Index coalesce

Question

hi masters.,
this is egarding index management. i am facing the problem regarding one sql query with union and order by clause.
when i remove order by clause, the query runs faster,than with order by clause.
is this the problem regarding index????
i know that indexes rarely need rebuild,thats why i am thinking of coalesce.is this the right decision??
what may be the problem else????
i am using oracle 9i. and i dont have statpack report coz the client is offshore.\
thanks and regards
VD

Answer 1

Vikrant,
Index is not picked by order by clause. Index entries are already sorted. See here a demo where I have used the index in a column.If I give order by clause, I explicitly get a Sort Order By in the plan.
Connected to:
Oracle Database 11g Enterprise Edition Release 11.1.0.6.0 - Production
With the Partitioning, Oracle Label Security, OLAP, Data Mining,
Oracle Database Vault and Real Application Testing options
SQL> create table sampletable as select * from scott.emp;
Table created.
SQL> create index idx on sampletable(empno);
Index created.
SQL> exec dbms_stats.gaher_table_stats(
BEGIN dbms_stats.gaher_table_stats(; END;
ERROR at line 1:
ORA-06550: line 1, column 36:
PLS-00103: Encountered the symbol ";" when expecting one of the following:
( ) - + case mod new not null <an identifier>
<a double-quoted delimited-identifier> <a bind variable>
table continue avg count current exists max min prior sql
stddev sum variance execute multiset the both leading
trailing forall merge year month day hour minute second
timezone_hour timezone_minute timezone_region timezone_abbr
time timestamp interval date
<a string literal with character set specification>
SQL> exec dbms_stats.gaher_table_stats('AMAN','SAMPLETABLE');
BEGIN dbms_stats.gaher_table_stats('AMAN','SAMPLETABLE'); END;
ERROR at line 1:
ORA-06550: line 1, column 18:
PLS-00302: component 'GAHER_TABLE_STATS' must be declared
ORA-06550: line 1, column 7:
PL/SQL: Statement ignored
SQL> exec dbms_stats.gather_table_stats('AMAN','SAMPLETABLE');
PL/SQL procedure successfully completed.
SQL> set autot trace exp
SQL> select * from sampletable order by empno;
Execution Plan
Plan hash value: 5644182
| Id  | Operation          | Name        | Rows  | Bytes | Cost (%CPU)| Time
|
|   0 | SELECT STATEMENT   |             |    14 |   504 |     4  (25)| 00:00:01
|
|   1 |  SORT ORDER BY     |             |    14 |   504 |     4  (25)| 00:00:01
|
|   2 |   TABLE ACCESS FULL| SAMPLETABLE |    14 |   504 |     3   (0)| 00:00:01
|
SQL> select * from sampletable order by empno where empno in (7369,7422);
select * from sampletable order by empno where empno in (7369,7422)
ERROR at line 1:
ORA-00933: SQL command not properly ended
SQL> select * from sampletable  where empno in (7369,7422);
Execution Plan
Plan hash value: 3966318501
| Id  | Operation                    | Name        | Rows  | Bytes | Cost (%CPU)
| Time     |
|   0 | SELECT STATEMENT             |             |     2 |    72 |     1   (0)
| 00:00:01 |
|   1 |  INLIST ITERATOR             |             |       |       |
|          |
|   2 |   TABLE ACCESS BY INDEX ROWID| SAMPLETABLE |     2 |    72 |     1   (0)
| 00:00:01 |
|*  3 |    INDEX RANGE SCAN          | IDX         |     2 |       |     0   (0)
| 00:00:01 |
Predicate Information (identified by operation id):
   3 - access("EMPNO"=7369 OR "EMPNO"=7422)
SQL> select * from sampletable  where empno in (7369,7422) order by empno;
Execution Plan
Plan hash value: 3966318501
| Id  | Operation                    | Name        | Rows  | Bytes | Cost (%CPU)
| Time     |
|   0 | SELECT STATEMENT             |             |     2 |    72 |     1   (0)
| 00:00:01 |
|   1 |  INLIST ITERATOR             |             |       |       |
|          |
|   2 |   TABLE ACCESS BY INDEX ROWID| SAMPLETABLE |     2 |    72 |     1   (0)
| 00:00:01 |
|*  3 |    INDEX RANGE SCAN          | IDX         |     2 |       |     0   (0)
| 00:00:01 |
Predicate Information (identified by operation id):
   3 - access("EMPNO"=7369 OR "EMPNO"=7422)
SQL>The point that I was making is that it may be happening that you are doing an over sort sort of thing which is causing the query to be slow. Using index for returning rows in the ordered manner is not a good thing as Oracle always say that for a guaranteed ordered result, one must use an order by clause. So either you can choose to remove the order by clause or can choose to rewrite the query without using the Union operator.
But these all are just guess works. Its not possible to say what's happening specifically in your case as long as you don't post the plan for your query both,with and without the use of index.
HTH
Aman....