$rand inside $facet is incorrectly cached by $lookup

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    • Type: Bug
    • Resolution: Unresolved
    • Priority: Major - P3
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    • Affects Version/s: None
    • Component/s: None
    • Query Optimization
    • ALL
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      db.a.drop()
db.a.insertMany([{}, {}])
db.a.aggregate([{$lookup: {from: "a", pipeline: [{$facet: {f: [{$set: {r: {$rand: {}}}}]}}], as: "res"}}])

      This produces something like:

      [
  {
    _id: ObjectId('6aaabda0460aabd8d9d1a7bd'),
    res: [
      {
        f: [
          { _id: ObjectId('6aaabda0460aabd8d9d1a7bd'), r: 0.9609455310720872 },
          { _id: ObjectId('6aaabda0460aabd8d9d1a7be'), r: 0.6046304842407205 }
        ]
      }
    ]
  },
  {
    _id: ObjectId('6aaabda0460aabd8d9d1a7be'),
    res: [
      {
        f: [
          { _id: ObjectId('6aaabda0460aabd8d9d1a7bd'), r: 0.9609455310720872 },
          { _id: ObjectId('6aaabda0460aabd8d9d1a7be'), r: 0.6046304842407205 }
        ]
      }
    ]
  }
]

      Note that the chosen random numbers are identical for both outer documents.

      For comparison, here is the same query without $facet and the output:

      db.a.aggregate([{$lookup: {from: "a", pipeline: [{$set: {r: {$rand: {}}}}], as: "res"}}])
      [
  {
    _id: ObjectId('6aaabda0460aabd8d9d1a7bd'),
    res: [
      { _id: ObjectId('6aaabda0460aabd8d9d1a7bd'), r: 0.48796429148767273 },
      { _id: ObjectId('6aaabda0460aabd8d9d1a7be'), r: 0.37433400902710473 }
    ]
  },
  {
    _id: ObjectId('6aaabda0460aabd8d9d1a7be'),
    res: [
      { _id: ObjectId('6aaabda0460aabd8d9d1a7bd'), r: 0.591967270406342 },
      { _id: ObjectId('6aaabda0460aabd8d9d1a7be'), r: 0.5688078786300821 }
    ]
  }
]
      Show
      db.a.drop() db.a.insertMany([{}, {}]) db.a.aggregate([{$lookup: {from: "a" , pipeline: [{$facet: {f: [{$set: {r: {$rand: {}}}}]}}], as: "res" }}]) This produces something like: [   {     _id: ObjectId('6aaabda0460aabd8d9d1a7bd'),     res: [       {         f: [           { _id: ObjectId('6aaabda0460aabd8d9d1a7bd'), r: 0.9609455310720872 },           { _id: ObjectId('6aaabda0460aabd8d9d1a7be'), r: 0.6046304842407205 }         ]       }     ]   },   {     _id: ObjectId('6aaabda0460aabd8d9d1a7be'),     res: [       {         f: [           { _id: ObjectId('6aaabda0460aabd8d9d1a7bd'), r: 0.9609455310720872 },           { _id: ObjectId('6aaabda0460aabd8d9d1a7be'), r: 0.6046304842407205 }         ]       }     ]   } ] Note that the chosen random numbers are identical for both outer documents. For comparison, here is the same query without $facet and the output: db.a.aggregate([{$lookup: {from: "a" , pipeline: [{$set: {r: {$rand: {}}}}], as: "res" }}]) [   {     _id: ObjectId('6aaabda0460aabd8d9d1a7bd'),     res: [       { _id: ObjectId('6aaabda0460aabd8d9d1a7bd'), r: 0.48796429148767273 },       { _id: ObjectId('6aaabda0460aabd8d9d1a7be'), r: 0.37433400902710473 }     ]   },   {     _id: ObjectId('6aaabda0460aabd8d9d1a7be'),     res: [       { _id: ObjectId('6aaabda0460aabd8d9d1a7bd'), r: 0.591967270406342 },       { _id: ObjectId('6aaabda0460aabd8d9d1a7be'), r: 0.5688078786300821 }     ]   } ]
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      Per SERVER-49024, $lookup should not cache subpipelines with $rand, but it misses $rand-s inside $facet-s.

            Assignee:
            Unassigned
            Reporter:
            Kartal Kaan Bozdogan
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              Created:
              Updated: