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https://issues.apache.org/jira/browse/IMPALA-8220?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel
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Paul Rogers reassigned IMPALA-8220:
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Assignee: (was: Paul Rogers)
> Track adjusted NDV for each column through plan tree
> ----------------------------------------------------
>
> Key: IMPALA-8220
> URL: https://issues.apache.org/jira/browse/IMPALA-8220
> Project: IMPALA
> Issue Type: Improvement
> Components: Frontend
> Affects Versions: Impala 3.1.0
> Reporter: Paul Rogers
> Priority: Minor
>
> See IMPALA-8213. To proper account for join cardinalities, we must track the
> adjusted NDV of columns as they pass through filters. IMPALA-8014,
> IMPALA-8015 and IMPALA-8213 suggest work-arounds based on the current code
> design.
> A better longer-term solution is to track the adjusted NDV for each column up
> the plan tree.
> That is, suppose we have column {{c}} with an original NDV of {{|c|}}. The
> scan applies a filter of {{c = 10}}. Clearly, the NDV out of the scan,
> {{|c'|}} is just 1.
> By tracking the filtered NDV, calculations up the tree become local. At
> present, the join node must reach down through the tree to find filters and
> potentially reverse them. This is complex and can be replaced with per-column
> NDV tracking.
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