In (select distinct .... ) Vs exists

Question

Guys, which would be less costly and also quicker performance:
SELECT a.*
          FROM table1 a
WHERE a.linkid IN (
          SELECT DISTINCT b.id1
                     FROM table2 b
                    WHERE b.REF = 'SOMETHING'
          UNION ALL
          SELECT DISTINCT b.id2
                     FROM table2 b
                    WHERE b.REF = 'SOMETHING'
          UNION ALL
          SELECT DISTINCT b.id3
                     FROM table2 b
                    WHERE b.REF = 'SOMETHING'
          UNION ALL
          SELECT DISTINCT b.id4
                     FROM table2 b
                    WHERE b.REF = 'SOMETHING'
          UNION ALL
          SELECT DISTINCT b.id5
                     FROM table2 b
                    WHERE b.REF = 'SOMETHING'
          UNION ALL
          SELECT DISTINCT b.id5
                     FROM table2 b
                    WHERE b.REF = 'SOMETHING'
          UNION ALL
          SELECT DISTINCT b.id7
                     FROM table2 b
                    WHERE b.REF = 'SOMETHING')
SELECT a.*
  FROM table1 a
WHERE EXISTS (
          SELECT 1
            FROM table2 b
           WHERE b.REF = 'SOMETHING'
             AND (   a.linkid = b.id1
                  OR a.linkid = b.id2
                  OR a.linkid = b.id3
                  OR a.linkid = b.id4
                  OR a.linkid = b.id5
                  OR a.linkid = b.id6
                  OR a.linkid = b.id7)
                   )I have a feeling the optimiser may execute both with the same plan.
Thanks, FIXXXER
Edited by: fixxxer on 04-Nov-2011 05:54

Answer 1

Depends on a lot of things, one of them being your version.
It's possible Oracle would rewrite your query using a myriad of techniques, one of the newer ones being an OR EXPANSION
http://blogs.oracle.com/optimizer/entry/or_expansion_transformation
One thing to note is that Oracle knows not to add the DISTINCT's inside of the IN clause .... because when you say IN (some_set) that set is always DISTINCT'd for you.