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https://issues.apache.org/jira/browse/TRAFODION-3031?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel
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David Wayne Birdsall resolved TRAFODION-3031.
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Resolution: Fixed
Fix Version/s: 2.3
> 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
> Priority: Major
> Fix For: 2.3
>
> 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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