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https://issues.apache.org/jira/browse/HIVE-10163?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14393406#comment-14393406
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Gunther Hagleitner commented on HIVE-10163:
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Alright. Small fix - had to re-add the checking for comparators just before
usage. Still - this is [~gopalv]'s code now - so I can +1 it.
> CommonMergeJoinOperator calls WritableComparator.get() in the inner loop
> ------------------------------------------------------------------------
>
> Key: HIVE-10163
> URL: https://issues.apache.org/jira/browse/HIVE-10163
> Project: Hive
> Issue Type: Bug
> Components: Query Processor
> Affects Versions: 1.2.0
> Reporter: Gopal V
> Assignee: Gunther Hagleitner
> Labels: JOIN, Performance
> Attachments: HIVE-10163.1.patch, HIVE-10163.2.patch,
> HIVE-10163.3.patch, mergejoin-comparekeys.png, mergejoin-parallel-bt.png,
> mergejoin-parallel-lock.png
>
>
> The CommonMergeJoinOperator wastes CPU looking up the correct comparator for
> each WritableComparable in each row.
> {code}
> @SuppressWarnings("rawtypes")
> private int compareKeys(List<Object> k1, List<Object> k2) {
> int ret = 0;
> ....
> ret = WritableComparator.get(key_1.getClass()).compare(key_1, key_2);
> if (ret != 0) {
> return ret;
> }
> }
> {code}
> !mergejoin-parallel-lock.png!
> !mergejoin-comparekeys.png!
> The slow part of that get() is deep within {{ReflectionUtils.setConf}}, where
> it tries to use reflection to set the Comparator config for each row being
> compared.
> !mergejoin-parallel-bt.png!
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