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  1. Core Server
  2. SERVER-79765

$match gets absorbed into ixscan when preceded by $group on $meta with accumulator on $meta

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    • Type: Icon: Bug Bug
    • Resolution: Works as Designed
    • Priority: Icon: Major - P3 Major - P3
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    • Query Integration
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      1. Run a timeseries aggregation with the pipeline 

      pipeline = [{$group: {_id: "$meta", x: {$min: "$meta"}}}, {$match: {_id: 1}}]

       2. Print out the result of the optimized pipeline 

      coll.explain().aggregate(pipeline)
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      1. Run a timeseries aggregation with the pipeline  pipeline = [{$group: {_id: "$meta" , x: {$min: "$meta" }}}, {$match: {_id: 1}}]  2. Print out the result of the optimized pipeline  coll.explain().aggregate(pipeline)
    • QI 2023-09-04
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      In a timeseries pipeline, when we first have a $group stage on the $meta field and an accumulator on the $min as the first stage, and then have a $match on the _id as the second stage, the pipeline is rewritten to absorb the $match into the index scan and leave the $group stage. 

      In essence, the stages $_unpack -> $group(_id=$meta, min=$meta) -> $match (_id=1) gets reordered to $IXSCAN (with $meta=1) -> $group(_id=$meta, min=$meta)

       

            Assignee:
            gil.alon@mongodb.com Gil Alon
            Reporter:
            alice.li@mongodb.com Alice Li (Inactive)
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