[jira] [Resolved] (HIVE-26184) COLLECT_SET with GROUP BY is very slow when some keys are highly skewed

["Zoltan Haindrich (Jira)" <ji...@apache.org>]

     [ https://issues.apache.org/jira/browse/HIVE-26184?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel ]

Zoltan Haindrich resolved HIVE-26184.
-------------------------------------
    Fix Version/s: 4.0.0-alpha-2
       Resolution: Fixed

merged into master. Thank you [~okumin] !

> 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
>              Labels: pull-request-available
>             Fix For: 4.0.0-alpha-2
>
>          Time Spent: 1.5h
>  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 table_with_many_rows
> 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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