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https://issues.apache.org/jira/browse/HIVE-15796?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15873848#comment-15873848
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Xuefu Zhang commented on HIVE-15796:
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Thanks for working on this, Chao! Above there is a TestSparkCliDriver test
failure and I'm wondering if it's related. Secondly, it looks like we added one
more path when setting reducer parallelism. Please explain a bit. (I remember
we didn't have that in a previous patch that I reviewed.) Using two passes to
avoid plan change, as you noted previously?
> HoS: poor reducer parallelism when operator stats are not accurate
> ------------------------------------------------------------------
>
> Key: HIVE-15796
> URL: https://issues.apache.org/jira/browse/HIVE-15796
> Project: Hive
> Issue Type: Improvement
> Components: Statistics
> Affects Versions: 2.2.0
> Reporter: Chao Sun
> Assignee: Chao Sun
> 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 operator;
> 2. when computing the parallelism for a ReduceWork, use the *maximum*
> parallelism from all its parents.
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