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From "Chao Sun (JIRA)" <>
Subject [jira] [Updated] (HIVE-15796) HoS: poor reducer parallelism when operator stats are not accurate
Date Wed, 22 Feb 2017 17:32:44 GMT


Chao Sun updated HIVE-15796:
       Resolution: Fixed
    Fix Version/s: 2.2.0
           Status: Resolved  (was: Patch Available)

Thanks [~xuefuz] for the review! I just committed this to the master branch.

> HoS: poor reducer parallelism when operator stats are not accurate
> ------------------------------------------------------------------
>                 Key: HIVE-15796
>                 URL:
>             Project: Hive
>          Issue Type: Improvement
>          Components: Statistics
>    Affects Versions: 2.2.0
>            Reporter: Chao Sun
>            Assignee: Chao Sun
>             Fix For: 2.2.0
>         Attachments: HIVE-15796.1.patch, HIVE-15796.2.patch, HIVE-15796.3.patch, HIVE-15796.4.patch,
HIVE-15796.5.patch, HIVE-15796.6.patch, HIVE-15796.wip.1.patch, HIVE-15796.wip.2.patch, HIVE-15796.wip.patch
> In HoS we use currently use operator stats to determine reducer parallelism. However,
it is often the case that operator stats are not accurate, especially if column stats are
not available. This sometimes will generate extremely poor reducer parallelism, and cause
HoS query to run forever. 
> This JIRA tries to offer an alternative way to compute reducer parallelism, similar to
how MR does. Here's the approach we are suggesting:
> 1. when computing the parallelism for a MapWork, use stats associated with the TableScan
> 2. when computing the parallelism for a ReduceWork, use the *maximum* parallelism from
all its parents.

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