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https://issues.apache.org/jira/browse/DRILL-6755?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16630910#comment-16630910
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ASF GitHub Bot commented on DRILL-6755:
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Ben-Zvi commented on issue #1480: DRILL-6755: Avoid building Hash Table for
inner/left join when probe side is empty
URL: https://github.com/apache/drill/pull/1480#issuecomment-425207228
Indeed this PR only addresses the "NONE 1st" case. As for the "NONE 2nd"
case (following a 1st OK_NEW_SCHEMA) - this would open up the can of worms we
saw before - early sniffing the probe side cranks the actual execution there
(e.g., another Hash Join below an Exchange) while other parts of the query are
still in schema discovery.
Not sure if in all cases where an operator finds it has no data to return
during schema discovery, a NONE is returned first. Note that the NONE does have
a schema (e.g., so a Right Join can return nulls for those missing columns).
Maybe we can increase the impact of this PR by checking all these cases, and
documenting/testing that our protocol allows for a NONE to be returned first
(i.e. during schema discovery).
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> HashJoin should not build hash tables when probe side is empty.
> ---------------------------------------------------------------
>
> Key: DRILL-6755
> URL: https://issues.apache.org/jira/browse/DRILL-6755
> Project: Apache Drill
> Issue Type: Improvement
> Reporter: Timothy Farkas
> Assignee: Boaz Ben-Zvi
> Priority: Major
>
> Currently when doing an Inner or a Right join we still build hashtables when
> the probe side is empty. A performance optimization would be to not build
> them.
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