Help with table scan

Question

I have a problem with full table scans that make very slow the performance of a report.
The test case is below. It looks that when the column is called from the table, the index is in use. If I use the same select from the view, then I get a table scan.
I would appreciate any idea on how to optimize it.
Thanks a lot for the help.
mj
<pre>
create table test1 (id1 number , id2 number, id3 number, col1 varchar(10),col2 varchar(50), col3 varchar(100));
create table test2 (id4 number , id5 number, id6 number, col4 varchar(10),col5 varchar(50), col6 varchar(100));
ALTER TABLE test1 ADD CONSTRAINT PK_test1 PRIMARY KEY(ID1) USING INDEX REVERSE;
create index index1 on test1(ID2);
create index index2 on test1(ID3,col2 );
ALTER TABLE test2 ADD CONSTRAINT PK_test2 PRIMARY KEY(ID4) USING INDEX REVERSE;
create or replace view test_view as select t1.*,
case (select t2.id4 from test2 t2 where t1.id2 = t2.id5 and t2.id6 = -1)
when t1.id2 then t1.id3
else t1.id2
end as main_id
from test1 t1 ;
create or replace view test_view2 as select * from test_view; --(requred by security levels)
select * from test1 where id2 =1000;
select * from test_view where id2 = 1000;
select * from test_view2 where id2 = 1000;
SQL> select * from test_view where id2 = 1000;
Elapsed: 00:00:00.00
Execution Plan
Plan hash value: 1970977999
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
| 0 | SELECT STATEMENT | | 1 | 125 | 1 (0)| 00:00:01 |
|* 1 | TABLE ACCESS FULL | TEST2 | 1 | 39 | 2 (0)| 00:00:01 |
| 2 | TABLE ACCESS BY INDEX ROWID| TEST1 | 1 | 125 | 1 (0)| 00:00:01 |
|* 3 | INDEX RANGE SCAN | INDEX1 | 1 | | 1 (0)| 00:00:01 |
Predicate Information (identified by operation id):
1 - filter("T2"."ID5"=:B1 AND "T2"."ID6"=(-1))
3 - access("T1"."ID2"=1000)
SQL> select * from test_view where main_id = 1000;
Elapsed: 00:00:00.03
Execution Plan
Plan hash value: 3806368241
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
| 0 | SELECT STATEMENT | | 1 | 125 | 4 (0)| 00:00:01 |
|* 1 | TABLE ACCESS FULL | TEST2 | 1 | 39 | 2 (0)| 00:00:01 |
|* 2 | FILTER | | | | | |
| 3 | TABLE ACCESS FULL| TEST1 | 1 | 125 | 2 (0)| 00:00:01 |
|* 4 | TABLE ACCESS FULL| TEST2 | 1 | 39 | 2 (0)| 00:00:01 |
Predicate Information (identified by operation id):
1 - filter("T2"."ID5"=:B1 AND "T2"."ID6"=(-1))
2 - filter(CASE WHEN "T1"."ID2"= (SELECT /*+ */ "T2"."ID4" FROM
MJ42."TEST2" "T2" WHERE "T2"."ID5"=:B1 AND "T2"."ID6"=(-1)) THEN
"T1"."ID3" ELSE "T1"."ID2" END =1000)
4 - filter("T2"."ID5"=:B1 AND "T2"."ID6"=(-1))
SQL>
</pre>

Answer 1

If you think about what the two queries are doing, it is easy to see why the first uses an index and the second does not.
Your first query:
SELECT * FROM test_view WHERE id2 = 1000explicitly uses an indexed column from test1 in the predicate. Oracle can use the index to identify the correct row from test1. Having found that single row in test1, it uses the FULL SCAN test2 to resolve the case statement.
Your second query:
SELECT * FROM test_view WHERE main_id = 1000uses the result of the case statement as the predicate. Oracle has no way of determing what row from test1 to use initially, so it must full scan both tables.
John