Excluding slow table access in a UNION ALL view

Question

Hi,
I have a view which unions three tables together.
One component of the view requires a table scan, as 90% of the records are required.
This view is then used in another outer select where these records are actually not required.
So I tagged each component with a code and excluded that in the outer select. However it still appears to access the table.
Is there any way I can exclude a component of the UNION ALL or do I need to explicitly split them?
example:
SELECT * FROM (
SELECT 'A' Q, JANUARY F FROM ENORMOUS.TABLE WHERE KEY = 'Non Selective Key' UNION ALL
SELECT 'B' Q, 1 F FROM DUAL UNION ALL
SELECT 'C' Q, 1 F FROM DUAL
) A
WHERE Q = 'B'
When I run the query plan without the WHERE it performs the table scan
When I include the WHERE it still performs the table scan but with a FILTER NULL IS NOT NULL afterwards.
So it appears that it is doing the table scan regardless and then throwing the records away - is that correct?
Any thoughts appreciated. I would prefer not the split this view out if possible as it is used everywhere.

Answer 1

In summary my question is: is The Oracle query planner smart enough to exclude a component from a load of stacked UNION ALL queries?
given this query:
CREATE VIEW TEST AS
SELECT Q, F
FROM
SELECT 'A' Q, JANUARY F FROM ENORMOUS.TABLE WHERE KEY = 'Non Selective Key' UNION ALL
SELECT 'B' Q, 1 F FROM DUAL UNION ALL
SELECT 'C' Q, 1 F FROM DUAL
) A;
-- 1. This one selects from all tables, including a table scan on the enormous table
SELECT * FROM TEST;
-- 2. This one selects from all tables, including a table scan on the enormous table
-- However the query plan has a FILTER after the table scan. Does it exclude this work?
SELECT * FROM TEST WHERE Q = 'B';When I run the query plan without the WHERE it performs the table scan
When I include the WHERE the query plan indicates it's doing the table scan but with a FILTER NULL IS NOT NULL afterwards.
So it appears that it is doing the table scan regardless and then throwing the records away - is that correct?
Any thoughts appreciated. I would prefer not the split this view out if possible as it is used everywhere.
Oracle version:
Oracle Database 11g Enterprise Edition Release 11.2.0.2.0 - 64bit Production
PL/SQL Release 11.2.0.2.0 - Production
CORE     11.2.0.2.0     Production
TNS for 64-bit Windows: Version 11.2.0.2.0 - Production
NLSRTL Version 11.2.0.2.0 - Production
Plan output for 1:
SELECT STATEMENT Optimizer Mode=ALL_ROWS (Cost=7166 Card=2 M Bytes=41 M)
1 VIEW (Cost=7166 Card=2 M Bytes=41 M)
2 1 UNION-ALL
3 2 TABLE ACCESS FULL PLANNING.BF_GEN_STATS_TRAN (Cost=7162 Card=2 M Bytes=72 M)
4 2 FAST DUAL (Cost=2 Card=1)
5 2 FAST DUAL (Cost=2 Card=1)
Plan output for 2:
SELECT STATEMENT Optimizer Mode=ALL_ROWS (Cost=2 Card=3 Bytes=48)
1 VIEW (Cost=2 Card=3 Bytes=48)
2 1 UNION-ALL
3 2 FILTER
4 3 TABLE ACCESS FULL PLANNING.BF_GEN_STATS_TRAN (Cost=7162 Card=2 M Bytes=72 M)
5 2 FAST DUAL (Cost=2 Card=1)
6 2 FILTER
7 6 FAST DUAL (Cost=2 Card=1)