How can i imrove peroformance of complex Query

Question

How can i improve performance of complex Query
which used CASE WHEN , GROUP By Rollup
for example
SELECT CASE WHEN null then a
ELSE b END
FROM (
SELECT a,b,c,..........
FROM table1
UNION ALL
SELECT a,b,c,..........
FROM table2
GROUP BY ROLLUP a,b,c ,............
this query take 18 min to return 180000 record
is there is any way to improve the performance of this query

Answer 1

Hello
Have you considered using
ALTER SYSTEM SET _fast=TRUE;-)
Alternatively you could provide some more information such as an execution plan for your query:
Oracle9i Enterprise Edition Release 9.2.0.4.0 - Production
With the Partitioning, OLAP and Oracle Data Mining options
JServer Release 9.2.0.4.0 - Production
SQL> set linesize 200
SQL> set pages 50000
SQL> EXPLAIN PLAN FOR
  2  select * from dual;
Explained.
SQL> select * from table(dbms_xplan.display);
PLAN_TABLE_OUTPUT
| Id  | Operation            |  Name       | Rows  | Bytes | Cost  |
|   0 | SELECT STATEMENT     |             |       |       |       |
|   1 |  TABLE ACCESS FULL   | DUAL        |       |       |       |
Note: rule based optimization
9 rows selected.If you want general info on tuning, I suggest you read the Perfomance tuning guide and the Data Warehousing guide. These are the best place to find information about concepts and techniques. The forums are better suited to specific questions.
HTH
David