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https://issues.apache.org/jira/browse/HIVE-17979?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel
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Ashutosh Chauhan updated HIVE-17979:
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Status: Open (was: Patch Available)
> Tez: Improve ReduceRecordSource passDownKey copying
> ---------------------------------------------------
>
> Key: HIVE-17979
> URL: https://issues.apache.org/jira/browse/HIVE-17979
> Project: Hive
> Issue Type: Improvement
> Affects Versions: 3.0.0
> Reporter: Gopal V
> Assignee: Gopal V
> Priority: Major
> Attachments: HIVE-17979.1.patch, HIVE-17979.2.patch
>
>
> Tez does not use a single Key stream for both sides of the join, so each
> input gets its own ReduceRecordSource
> {code}
> sources[tag] = new ReduceRecordSource();
> {code}
> And this means for each input stream, there's a deserialized key (because the
> tag is not part of the Key byte stream), this means for a 2-table join there
> are 2 ReduceRecordSource objects.
> This means that the passDownKey is only an optimization when the Key,
> List<Value> has more than 1 value in it. Otherwise the copy is entirely
> wasted CPU cycles, because it deserializes the entire row to extract the key
> and discards the row.
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