HIVE-26184: COLLECT_SET with GROUP BY is very slow when some keys are highly skewed#3253
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CI failed but I think it's not apparently caused by this PR. |
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DongWei-4
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… highly skewed (apache#3253) (okumin reviewed by Zoltan Haindrich)
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… highly skewed (apache#3253) (okumin reviewed by Zoltan Haindrich)
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What changes were proposed in this pull request?
This would reduce the time complexity of
COLLECT_SETfromO({maximum length} * {num rows})intoO({maximum length} + {num rows}).https://issues.apache.org/jira/browse/HIVE-26184
Why are the changes needed?
I'm observing some reducers take much time due to this issue.
Does this PR introduce any user-facing change?
No
How was this patch tested?
I have run the reproduction case in HIVE-26184 with this patch and confirmed the reduce vertex finished more than 30x faster.