Oracle EXISTS query is not working as expected - DB Link bug?

Question

I'm completely baffled by the results of this query:

select count(*) from my_tab mt
where  mt.stat = '2473'
  and mt.name= 'Tom'
  and exists (select * from [email protected] cu, 
                [email protected] pr
              where cu.user_id = pr.user_id
              and mt.name = pr.name
              and mt.stat = cu.stat
              )

Returns: 1

There are 0 records in [email protected] with stat='2473', so why is it returning true for the exists?

If I change the query like so, it returns 0:

select count(*) from my_tab mt
where  mt.stat = '2473'
  and mt.name= 'Tom'
  and exists (select * from [email protected] cu, 
                [email protected] pr
              where cu.user_id = pr.user_id
              and mt.name = pr.name
              and cu.stat = '2473'
              )

UPDATE Okay, this is really weird. Just to see what would happen, I executed the query from the other database (the one referenced by the DB Links) and it gave different (correct) results.

select count(*) from [email protected] mt
    where  mt.stat = '2473'
      and mt.name= 'Tom'
      and exists (select * from company_users cu, 
                    personnel_records pr
                  where cu.user_id = pr.user_id
                  and mt.name = pr.name
                  and mt.stat = cu.stat
                  )

Returns 0 (as expected).

Answer 1

Have a look at the explain plan for the first query. I suspect there is a bug, and the query plan may show how an invalid rewrite is being done.