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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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    • Backwards Compatibility:
      Fully Compatible

      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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          Hide
          samuelgmartinez Samuel García Martínez added a comment - - edited

          Since in this case mongos regrouping process isn't needed, mongos shouldn't fetch entire resultset, sending limit/skip to shards (if there is no sort operation).

          Show
          samuelgmartinez Samuel García Martínez added a comment - - edited Since in this case mongos regrouping process isn't needed, mongos shouldn't fetch entire resultset, sending limit/skip to shards (if there is no sort operation).
          Hide
          samuelgmartinez Samuel García Martínez added a comment -

          I developed a fix for this issue. Is there any process or prerrequisites to do a pull request on Github with this fix?

          To give this more accuracy, $group _id can be a superset of shardkey too.

          Show
          samuelgmartinez Samuel García Martínez added a comment - I developed a fix for this issue. Is there any process or prerrequisites to do a pull request on Github with this fix? To give this more accuracy, $group _id can be a superset of shardkey too.
          Hide
          ian.whalen Ian Whalen added a comment -

          @samuel, the first step is to fill out the contributor agreement - http://www.10gen.com/contributor - and then open a pull request at https://github.com/mongodb/mongo/pulls

          Show
          ian.whalen Ian Whalen added a comment - @samuel, the first step is to fill out the contributor agreement - http://www.10gen.com/contributor - and then open a pull request at https://github.com/mongodb/mongo/pulls
          Hide
          samuelgmartinez Samuel García Martínez added a comment -

          Hi. I submitted a pull request for this issue. I hope it helps.

          https://github.com/mongodb/mongo/pull/294

          Show
          samuelgmartinez Samuel García Martínez added a comment - Hi. I submitted a pull request for this issue. I hope it helps. https://github.com/mongodb/mongo/pull/294
          Hide
          asya Asya Kamsky added a comment -

          The same optimization can be extended if $group is done on the original document _id field (as would be the case if you $unwind and $group by _id to process some array in the document).

          Show
          asya Asya Kamsky added a comment - The same optimization can be extended if $group is done on the original document _id field (as would be the case if you $unwind and $group by _id to process some array in the document).
          Hide
          jon.rangel Jon Rangel added a comment -

          This optimization should also support hashed shard keys.

          e.g. if sharding on {foo:"hashed"}, grouping on "foo" should employ the optimization.

          Show
          jon.rangel Jon Rangel added a comment - This optimization should also support hashed shard keys. e.g. if sharding on { foo:"hashed" }, grouping on "foo" should employ the optimization.
          Hide
          charlie.swanson Charlie Swanson added a comment -

          We've taken a look at how we might achieve this, and the changes required are non-trivial. After some discussion, we've decided this will not be completed for 3.2, and we will re-prioritize when planning for the next release.

          What complicates this is that another stage such as a $project stage could modify the _id or shard key of a document, e.g. the $group stage could not be performed entirely on the shards for the following pipeline.

          db.coll.aggregate([
              {$project: {_id: {$literal: 1}}},
              {$group: {_id: '$_id'}}
          ])
          

          While this is still a very useful optimization, we believe several of the new expressions capable of operating on arrays available in the $project stage should reduce the need for unwinding and then regrouping, in turn reducing the need for this optimization. e.g. SERVER-9625, SERVER-4589, SERVER-8141, SERVER-10626, SERVER-14872.

          Show
          charlie.swanson Charlie Swanson added a comment - We've taken a look at how we might achieve this, and the changes required are non-trivial. After some discussion, we've decided this will not be completed for 3.2, and we will re-prioritize when planning for the next release. What complicates this is that another stage such as a $project stage could modify the _id or shard key of a document, e.g. the $group stage could not be performed entirely on the shards for the following pipeline. db.coll.aggregate([ {$project: {_id: {$literal: 1}}}, {$group: {_id: '$_id' }} ]) While this is still a very useful optimization, we believe several of the new expressions capable of operating on arrays available in the $project stage should reduce the need for unwinding and then regrouping, in turn reducing the need for this optimization. e.g. SERVER-9625 , SERVER-4589 , SERVER-8141 , SERVER-10626 , SERVER-14872 .

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              • Created:
                Updated:
                Days since reply:
                35 weeks, 5 days ago
                Date of 1st Reply:

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