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https://issues.apache.org/jira/browse/FLINK-14874?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel
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Kurt Young reassigned FLINK-14874:
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Assignee: godfrey he
> add local aggregate to solve data skew for ROLLUP/CUBE case
> -----------------------------------------------------------
>
> Key: FLINK-14874
> URL: https://issues.apache.org/jira/browse/FLINK-14874
> Project: Flink
> Issue Type: Sub-task
> Components: Table SQL / Planner
> Affects Versions: 1.9.1
> Reporter: godfrey he
> Assignee: godfrey he
> Priority: Major
> Labels: pull-request-available
> Fix For: 1.10.0
>
> Time Spent: 10m
> Remaining Estimate: 0h
>
> Many tpc-ds queries have {{rollup}} keyword, which will be translated to
> multiple groups.
> for example: {{group by rollup (channel, id)}} is equivalent {{group by
> (channel, id)}} + {{group by (channel)}} + {{group by ()}}.
> All data on empty group will be shuffled to a single node, It is a typical
> data skew case. If there is a local aggregate, the data size shuffled to the
> single node will be greatly reduced. However, currently the cost mode can't
> estimate the local aggregate's cost, and the plan with local aggregate may be
> chose even the query has {{rollup}} keyword.
> we could add a rule based phase (after physical phase) to enforce local
> aggregate if it's input has empty group.
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