okumin created HIVE-26184:
-----------------------------
Summary: 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: 3.1.3, 2.3.8
Reporter: okumin
Assignee: okumin
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}
--
This message was sent by Atlassian Jira
(v8.20.7#820007)
