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https://issues.apache.org/jira/browse/PIG-4449?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14349465#comment-14349465
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Daniel Dai commented on PIG-4449:
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Good optimization. POSort already capture "limit" semantics, we just need to
make use of it in POSort and enable it with an optimization rule.
> Optimize the case of Order by + Limit in nested foreach
> -------------------------------------------------------
>
> Key: PIG-4449
> URL: https://issues.apache.org/jira/browse/PIG-4449
> Project: Pig
> Issue Type: Improvement
> Reporter: Rohini Palaniswamy
>
> This is one of the very frequently used patterns
> {code}
> grouped_data_set = group data_set by id;
> capped_data_set = foreach grouped_data_set
> {
> ordered = order joined_data_set by timestamp desc;
> capped = limit ordered $num;
> generate flatten(capped);
> };
> {code}
> But this performs very poorly when there are millions of rows for a key in
> the groupby with lot of spills. This can be easily optimized by pushing the
> limit into the InternalSortedBag and maintain only $num records any time and
> avoid memory pressure.
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