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https://issues.apache.org/jira/browse/HIVE-6247?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel
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Gopal V resolved HIVE-6247.
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Resolution: Not A Problem
> select count(distinct) should be MRR in Tez
> -------------------------------------------
>
> Key: HIVE-6247
> URL: https://issues.apache.org/jira/browse/HIVE-6247
> Project: Hive
> Issue Type: Bug
> Components: Tez
> Affects Versions: 0.13.0
> Reporter: Gopal V
> Assignee: Gunther Hagleitner
>
> The MR query plan for "select count(distinct) " fires off multiple reducers,
> with a local work task to perform final aggregation.
> The Tez version fires off exactly 1 reducer for the entire data-set which
> chokes and dies/slows down massively.
> To reproduce on a TPC-DS database (meaningless query)
> {code}
> select count(distinct ss_net_profit) from store_sales ss join store s on
> ss.ss_store_sk = s.s_store_sk;
> {code}
> This spins up Map 1, Map 2 (for the dim table + fact table) & Reducer 1 which
> is always "0/1".
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