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  1. Documentation
  2. DOCS-15054

[Server] Investigate changes in SERVER-57383: Include merge sort in sharded aggregation explain output

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    • Task
    • Status: Open
    • Major - P3
    • Resolution: Unresolved
    • None
    • 5.3.0
    • manual, Server
    • None

    Description

      Downstream Change Summary

      This ticket changes explain output for queries against sharded collections, adding the $mergeCursors stage to the front of the mergerPart of the split pipeline.

      Description of Linked Ticket

      At some point before 4.2 we stopped including the merge sort in the explain output - I believe because the merge sort started happening within the $mergeCursors, which is not created nor displayed in explain output.

      We should somehow enhance the explain output to include the merge sort so that users don't mistakenly conclude that the merged output is not sorted.

       
      4.0:

      splitPipeline: {
        shardsPart: [
          {
            '$match': { mykey: { '$in': [ 0, 1, 3, 7 ] } }
          },
          { '$sort': { sortKey: { favColour: 1 }, limit: 5 } }
        ],
        mergerPart: [
          {
            '$sort': { sortKey: { favColour: 1 }, mergePresorted: true, limit: 5 }
          }
        ]
      }
      

      4.2/4.4:

       
      splitPipeline: {
        shardsPart: [
          {
            '$match': { mykey: { '$in': [ 0, 1, 3, 7 ] } }
          },
          { '$sort': { sortKey: { favColour: 1 }, limit: 5 } }
        ],
        mergerPart: [ { '$limit': 5 } ]
      }
      

       

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              Unassigned Unassigned
              backlog-server-pm Backlog - Core Eng Program Management Team
              Joseph Dougherty Joseph Dougherty
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              Dates

                Created:
                Updated:
                45 weeks, 5 days ago