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  1. Core Server
  2. SERVER-5477

when sharded, no need to merge groups if $group _id is the shard key or original document _id

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Details

    • Query Optimization
    • Fully Compatible
    • QuInt 8 08/28/15, Query 2019-06-17, Query 2019-07-01, Query 2019-07-15, Query 2019-07-29, Query 2019-08-12

    Description

      Copied from SERVER-4961:

      On sharded environment, using early grouping, besides the use of an index, it would be nice that we be able to avoid the mongos regrouping process.

      I'll try to explain that:

        * result_node1: [
      		 
           {
      		 
             id: "value1",
      		 
             totalcount: 50
      		 
           },
      		 
           {
      		 
             id: "value2",
      		 
             totalcount: 100
      		 
           },
      		 
         ]
      		 
       * result_node2: [
      		 
           {
      		 
             id: "value1",
      		 
             totalcount: 60
      		 
           }
      		 
         ]

      The real results(after mongos regroup) must looks like:

       [
      		 
           {
      		 
             id: "value1",
      		 
             totalcount: 110
      		 
           },
      		 
           {
      		 
             id: "value2",
      		 
             totalcount: 100
      		 
           },
      		 
       ]

      But, in some cases, mongos regrouping process is nonsense since the grouping key is same as sharding key. So, never got same group key from different shards.

      So, the prior example, now looks like:

        * result_node1: [
      		 
           {
      		 
             id: "value1",
      		 
             totalcount: 110
      		 
           }
      		 
         ]
      		 
       * result_node2: [
      		 
           {
      		 
             id: "value2",
      		 
             totalcount: 100
      		 
           }
      		 
         ]

      The real results must looks like:

       [
      		 
           {
      		 
             id: "value1",
      		 
             totalcount: 110
      		 
           },
      		 
           {
      		 
             id: "value2",
      		 
             totalcount: 100
      		 
           },
      		 
       ]

      So, the point is mongos regrouping process is a waste of time when you group using same key as sharding key.

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        Issue Links

          depends on

          Bug - A problem which impairs or prevents the functions of the product. SERVER-42160 $group stages that use a DISTINCT_SCAN do not use a SHARDING_FILTER on sharded collections

          • Major - P3 - Major loss of function.
          • Backlog

          Improvement - An improvement or enhancement to an existing feature or task. SERVER-41750 Refactor renamedPaths() helpers to support renames in either direction

          • Major - P3 - Major loss of function.
          • Closed
          is duplicated by

          Improvement - An improvement or enhancement to an existing feature or task. SERVER-22912 Query Optimizer

          • Trivial - P5 - Cosmetic problem like misspelt words or misaligned text.
          • Closed
          is related to

          Bug - A problem which impairs or prevents the functions of the product. SERVER-27115 Track fields renamed by $project in aggregation for index consideration

          • Major - P3 - Major loss of function.
          • Closed

          Improvement - An improvement or enhancement to an existing feature or task. SERVER-28942 sort by shard key or prefix of shard key may not require merge before group

          • Major - P3 - Major loss of function.
          • Backlog

          Improvement - An improvement or enhancement to an existing feature or task. SERVER-55200 DISTINCT_SCAN not used for $sort+$match+$group+$first on sharded collection

          • Minor - P4 - Minor loss of function, or other problem where easy workaround is present.
          • Backlog
          related to

          Improvement - An improvement or enhancement to an existing feature or task. SERVER-56583 Push $setWindowFields to shards when shards contain whole partitions

          • Minor - P4 - Minor loss of function, or other problem where easy workaround is present.
          • Backlog

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              backlog-query-optimization Backlog - Query Optimization
              dan@mongodb.com Daniel Pasette (Inactive)
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              Dates

                Created:
                Updated: