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https://issues.apache.org/jira/browse/HIVE-15796?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15880122#comment-15880122
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Lefty Leverenz commented on HIVE-15796:
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By the way, the multi-line description of *hive.spark.use.op.stats* should have
included newlines (\n) to avoid having a single-line description in the
generated template file hive-default.xml.template. (Several hive.spark.*
parameters make the same mistake in their descriptions, but
*hive.spark.dynamic.partition.pruning* gets it right.)
> 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
> Labels: TODOC2.2
> 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 operator;
> 2. when computing the parallelism for a ReduceWork, use the *maximum*
> parallelism from all its parents.
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