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https://issues.apache.org/jira/browse/HIVE-26184?focusedWorklogId=763368&page=com.atlassian.jira.plugin.system.issuetabpanels:worklog-tabpanel#worklog-763368
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ASF GitHub Bot logged work on HIVE-26184:
-----------------------------------------
Author: ASF GitHub Bot
Created on: 28/Apr/22 08:39
Start Date: 28/Apr/22 08:39
Worklog Time Spent: 10m
Work Description: okumin opened a new pull request, #3253:
URL: https://github.com/apache/hive/pull/3253
### What changes were proposed in this pull request?
This would reduce the time complexity of `COLLECT_SET` from `O({maximum
length} * {num rows})` into `O({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.
Issue Time Tracking
-------------------
Worklog Id: (was: 763368)
Remaining Estimate: 0h
Time Spent: 10m
> COLLECT_SET with GROUP BY is very slow when some keys are highly skewed
> -----------------------------------------------------------------------
>
> Key: HIVE-26184
> URL: https://issues.apache.org/jira/browse/HIVE-26184
> Project: Hive
> Issue Type: Bug
> Components: Hive
> Affects Versions: 2.3.8, 3.1.3
> Reporter: okumin
> Assignee: okumin
> Priority: Major
> Time Spent: 10m
> Remaining Estimate: 0h
>
> I observed some reducers spend 98% of CPU time in invoking
> `java.util.HashMap#clear`.
> Looking the detail, I found COLLECT_SET reuses a LinkedHashSet and its
> `clear` can be quite heavy when a relation has a small number of highly
> skewed keys.
>
> To reproduce the issue, first, we will create rows with a skewed key.
> {code:java}
> INSERT INTO test_collect_set
> SELECT '00000000-0000-0000-0000-000000000000' AS key, CAST(UUID() AS VARCHAR)
> AS value
> FROM table_with_many_rows
> LIMIT 100000;{code}
> Then, we will create many non-skewed rows.
> {code:java}
> INSERT INTO test_collect_set
> SELECT UUID() AS key, UUID() AS value
> FROM sample_datasets.nasdaq
> LIMIT 5000000;{code}
> We can observe the issue when we aggregate values by `key`.
> {code:java}
> SELECT key, COLLECT_SET(value) FROM group_by_skew GROUP BY key{code}
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