Using exists with dual

Question

I was execting the following query to be cost-effective but the the select from dual shows a cost of 17. The inner query accesses a view.
SELECT 1 into x
FROM DUAL
WHERE EXISTS (
SELECT 1
FROM data_v
WHERE project_id = 9031
AND empid = 497681
AND role = 'PROH')

Answer 1

I don't think there's a significant performance degradation with regards to the use of dual in that case. My assumption is that there must be some problem with the DATA_V view that needs optimization. If I were to change the subquery to something else, it would have a change in performance.
However, you may want to consider just reconstructing your query to this:
SELECT 1 into x
FROM data_v
WHERE project_id = 9031
AND empid = 497681
AND role = 'PROH'
AND rownum = 1; << the only thing I added to check just 1 record for existence
There won't be much performance gain if the DATA_V is not optimized. The change was just to remove the use of Dual and achieve the same results.