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https://issues.apache.org/jira/browse/HIVE-17896?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16391375#comment-16391375
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Teddy Choi commented on HIVE-17896:
-----------------------------------
This eighth patch fixes object inspector class cast error and applies recent
changes in the master branch.
> TopNKey: Create a standalone vectorizable TopNKey operator
> ----------------------------------------------------------
>
> Key: HIVE-17896
> URL: https://issues.apache.org/jira/browse/HIVE-17896
> Project: Hive
> Issue Type: New Feature
> Components: Operators
> Affects Versions: 3.0.0
> Reporter: Gopal V
> Assignee: Teddy Choi
> Priority: Major
> Attachments: HIVE-17896.1.patch, HIVE-17896.3.patch,
> HIVE-17896.4.patch, HIVE-17896.5.patch, HIVE-17896.6.patch,
> HIVE-17896.7.patch, HIVE-17896.8.patch
>
>
> For TPC-DS Query27, the TopN operation is delayed by the group-by - the
> group-by operator buffers up all the rows before discarding the 99% of the
> rows in the TopN Hash within the ReduceSink Operator.
> The RS TopN operator is very restrictive as it only supports doing the
> filtering on the shuffle keys, but it is better to do this before breaking
> the vectors into rows and losing the isRepeating properties.
> Adding a TopN Key operator in the physical operator tree allows the following
> to happen.
> GBY->RS(Top=1)
> can become
> TNK(1)->GBY->RS(Top=1)
> So that, the TopNKey can remove rows before they are buffered into the GBY
> and consume memory.
> Here's the equivalent implementation in Presto
> https://github.com/prestodb/presto/blob/master/presto-main/src/main/java/com/facebook/presto/operator/TopNOperator.java#L35
> Adding this as a sub-feature of GroupBy prevents further optimizations if the
> GBY is on keys "a,b,c" and the TopNKey is on just "a".
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