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From "Gopal V (JIRA)" <j...@apache.org>
Subject [jira] [Resolved] (HIVE-6247) select count(distinct) should be MRR in Tez
Date Tue, 08 Mar 2016 02:36:40 GMT

     [ https://issues.apache.org/jira/browse/HIVE-6247?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel
]

Gopal V resolved HIVE-6247.
---------------------------
    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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