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https://issues.apache.org/jira/browse/TRAFODION-2349?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15866712#comment-15866712
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Hans Zeller commented on TRAFODION-2349:
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Part of this was addressed by
https://github.com/apache/incubator-trafodion/pull/928. This fixed many issues
with single column stats, but there is still some work missing for multi-column
stats.
> Solve issue with mapping of value ids for statistics for common subexpr
> -----------------------------------------------------------------------
>
> Key: TRAFODION-2349
> URL: https://issues.apache.org/jira/browse/TRAFODION-2349
> Project: Apache Trafodion
> Issue Type: Sub-task
> Components: sql-cmp
> Affects Versions: 2.1-incubating
> Reporter: Hans Zeller
> Assignee: Hans Zeller
> Fix For: 2.2-incubating
>
>
> When we introduce a temp table, we want to "transplant" the statistics from
> the original query tree representing the common subexpression to the new scan
> and its ancestors. This requires two mappings, one from the original tree to
> the temp table columns, the other from the temp table cols to the
> characteristic outputs of the MapValueIds above the temp scan.
> While most of these mappings are done in terms of VEGRefs, the statistics use
> BaseColumns instead. Right now, we map the BaseColumns of single column stats
> to VEGRefs in the new tree. We might be able to do the same for MC stats.
> Or, alternatively, we may need to decided on specific BaseColumns to map to.
> See methods Scan::synthEstLogProp() and MapValueIds::synthEstLogProp().
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