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https://issues.apache.org/jira/browse/DRILL-6115?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16365069#comment-16365069
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ASF GitHub Bot commented on DRILL-6115:
---------------------------------------

Github user HanumathRao commented on the issue:

    https://github.com/apache/drill/pull/1110
  
    @amansinha100  @vrozov  Thanks for the review. I have squashed all the 
commits into two commits.
    
    Please merge these two commits individually into the apache master branch. 
First commit is for refactoring existing code. Second commit is about fixing 
this JIRA request.


> SingleMergeExchange is not scaling up when many minor fragments are allocated 
> for a query.
> ------------------------------------------------------------------------------------------
>
>                 Key: DRILL-6115
>                 URL: https://issues.apache.org/jira/browse/DRILL-6115
>             Project: Apache Drill
>          Issue Type: Improvement
>          Components: Execution - Relational Operators
>    Affects Versions: 1.12.0
>            Reporter: Hanumath Rao Maduri
>            Assignee: Hanumath Rao Maduri
>            Priority: Major
>             Fix For: 1.13.0
>
>         Attachments: Enhancing Drill to multiplex ordered merge exchanges.docx
>
>
> SingleMergeExchange is created when a global order is required in the output. 
> The following query produces the SingleMergeExchange.
> {code:java}
> 0: jdbc:drill:zk=local> explain plan for select L_LINENUMBER from 
> dfs.`/drill/tables/lineitem` order by L_LINENUMBER;
> +------+------+
> | text | json |
> +------+------+
> | 00-00 Screen
> 00-01 Project(L_LINENUMBER=[$0])
> 00-02 SingleMergeExchange(sort0=[0])
> 01-01 SelectionVectorRemover
> 01-02 Sort(sort0=[$0], dir0=[ASC])
> 01-03 HashToRandomExchange(dist0=[[$0]])
> 02-01 Scan(table=[[dfs, /drill/tables/lineitem]], 
> groupscan=[JsonTableGroupScan [ScanSpec=JsonScanSpec 
> [tableName=maprfs:///drill/tables/lineitem, condition=null], 
> columns=[`L_LINENUMBER`], maxwidth=15]])
> {code}
> On a 10 node cluster if the table is huge then DRILL can spawn many minor 
> fragments which are all merged on a single node with one merge receiver. 
> Doing so will create lot of memory pressure on the receiver node and also 
> execution bottleneck. To address this issue, merge receiver should be 
> multiphase merge receiver. 
> Ideally for large cluster one can introduce tree merges so that merging can 
> be done parallel. But as a first step I think it is better to use the 
> existing infrastructure for multiplexing operators to generate an OrderedMux 
> so that all the minor fragments pertaining to one DRILLBIT should be merged 
> and the merged data can be sent across to the receiver operator.
> On a 10 node cluster if each node processes 14 minor fragments.
> Current version of code merges 140 minor fragments
> the proposed version has two level merges 1 - 14 merge in each drillbit which 
> is parallel 
> and 10 minorfragments are merged at the receiver node.



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