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https://issues.apache.org/jira/browse/SOLR-8281?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15012646#comment-15012646
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Joel Bernstein commented on SOLR-8281:
--------------------------------------

Thinking about this some more, possibly this is a job for the ReducerStream. We 
could add Operations to the reducer stream and have the operations perform the 
merge.

In we went this route we would scrap the MergeRollupStream and change this 
ticket to "Add operations to the ReducerStream".

> Add RollupMergeStream to Streaming API
> --------------------------------------
>
>                 Key: SOLR-8281
>                 URL: https://issues.apache.org/jira/browse/SOLR-8281
>             Project: Solr
>          Issue Type: Bug
>            Reporter: Joel Bernstein
>            Assignee: Joel Bernstein
>
> The RollupMergeStream merges the aggregate results emitted by the 
> RollupStream on *worker* nodes.
> This is designed to be used in conjunction with the HashJoinStream to perform 
> rollup Aggregations on the joined Tuples. The HashJoinStream will require the 
> tuples to be partitioned on the Join keys. To avoid needing to repartition on 
> the *group by* fields for the RollupStream, we can perform a merge of the 
> rolled up Tuples coming from the workers.
> The construct would like this:
> {code}
> mergeRollup (...
>                       parallel (...
>                                     rollup (...
>                                                 hashJoin (
>                                                                   search(...),
>                                                                   search(...),
>                                                                   on="fieldA" 
>                                                 )
>                                      )
>                          )
>                )
> {code}
> The pseudo code above would push the *hashJoin* and *rollup* to the *worker* 
> nodes. The emitted rolled up tuples would be merged by the mergeRollup.
>  



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