IS NOT NULL and performance

Question

I have a performance issue with a query - it would be something like -
select col1,col2, sum(col3), get_val(col_4)
from table1
where
get_val(col_4) is not null
group by col1,col2, get_val(col_4)
I have simplified this but it is something similar. This works great - performance is great. Now I commented out the where clause as I needed to populate null values - and that's it the query does not retrieve the resultset - it keeps running forever. With the where clause it comes back in 60 seconds. There is only one row out of 560 rows that has null value for col_4 which i need to display.
Any help is appreciated.

Answer 1

The only difference I notice between the two sqls is HASH(UNIQUE) -
with IS NOT NULL in where clause -
SELECT STATEMENT     ALL_ROWS     1598     1     209
HASH(UNIQUE)          1598     1     209
HASH(GROUP BY)          1598     1     209
When is not null is removed from the where clause -
SELECT STATEMENT     ALL_ROWS     1598     1     206
HASH(GROUP BY)          1598     1     206          
I'm guessing that the index is being used in the first scenario and not in the second. Any idea/suggestion as to how to over come this?          
Thanks