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  1. Core Server
  2. SERVER-30474

leading $facet with each facet beginning with $match should add $match before $facet

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    • Query Optimization

      Imagine you have an aggregation pipeline that looks something like:

      db.collection.aggregate([
  {$facet: {
    xs: [
      {$match: {x: "SOMETHING"}},
      {$group: {...}}
    ],
    ys: [
      {$match: {y: "SOMETHING ELSE"}},
      {$unwind: "$y"},
      ...
    ]
  }}
])

      Such a pipeline would not provide any predicates for the query system to use to begin the pipeline. This pipeline might benefit from being optimized to something more like:

      db.collection.aggregate([
  {$match: {$or: [{x: "SOMETHING"}, {y: "SOMETHING ELSE"}]}},
  {$facet: {
    xs: [
      {$match: {x: "SOMETHING"}},
      {$group: {...}}
    ],
    ys: [
      {$match: {y: "SOMETHING ELSE"}},
      {$unwind: "$y"},
      ...
    ]
  }}
])
      Original Description

      $facet is not smart enough to use an index. if the only stage in a pipeline is $facet, the planning system will simply default to a COLLSCAN.

            Assignee:
            backlog-query-optimization [DO NOT USE] Backlog - Query Optimization
            Reporter:
            vick.mena Vick Mena (Inactive)
            Votes:
            9 Vote for this issue
            Watchers:
            28 Start watching this issue

              Created:
              Updated: