[jira] [Created] (TRAFODION-3031) Query with nested subqueries chooses bad plan

["David Wayne Birdsall (JIRA)" <ji...@apache.org>]

David Wayne Birdsall created TRAFODION-3031:
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             Summary: Query with nested subqueries chooses bad plan
                 Key: TRAFODION-3031
                 URL: https://issues.apache.org/jira/browse/TRAFODION-3031
             Project: Apache Trafodion
          Issue Type: Improvement
          Components: sql-cmp
    Affects Versions: 2.3
            Reporter: David Wayne Birdsall
            Assignee: David Wayne Birdsall
         Attachments: jira.log, jira.sql

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]



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