The attached files demonstrate the problem. The file jira.sql is a script that reproduces the problem, while jira.log is a sqlci showing the results.
The query in question does an IN-subquery from T1 to T2, then T2 has an =-subquery back to T1. T2 contains two indexes, one each on the join columns.
The default plan uses a hybrid hash join of T1 to T2 and is very slow. It does a full scan of both T1 and T2.
If we set CQD SEMIJOIN_TO_INNERJOIN_TRANSFORMATION 'ON', the plan is a little bit better. We get a nested join of T1 to T2. But it is inefficient; we still do a full scan of T2.
If we rename the index T2A to T2Y, and we still have the CQD set, we get a good nested join plan that uses the index T2Y and reads just one row at each level. This is very fast.
So, there are two issues here.
- We could do a better job of deciding when to do the semi-join to join transformation. When the inner table is small, it is profitable to do this.
- The index elimination logic is mistakenly eliminating index T2A so the Optimizer misses a chance to use it and so does not find the efficient nested join plan.jira.log