A later $sort does not reset {$meta:"sortKey"} when the two sorts cannot be coalesced

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    • Type: Bug
    • Resolution: Unresolved
    • Priority: Minor - P4
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    • Affects Version/s: None
    • Component/s: None
    • Query Optimization
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      // code placeholder
const c = db.getSiblingDB("sk_bug").c;
c.drop();
c.insertMany([
    {_id: 1, a: 3, b: "x"},
    {_id: 2, a: 1, b: "y"},
    {_id: 3, a: 2, b: "z"}]);

// Two sorts on different fields, separated by a no-op $limit so they cannot coalesce.
// Final order is by 'b' (x,y,z), so {$meta:"sortKey"} should reflect 'b'.
const docs = c.aggregate([
    {$sort: {a: 1}},
    {$limit: 1000000000}, // Any other stage would equally surface the issue. Just added this one as is a no-op, but any other else would also reproduce it
    {$sort: {b: 1}},
    {$project: {_id: 0, b: 1, sk: {$meta: "sortKey"}}},
]).toArray();

printjson(docs);

      Expected: the sort key reflects the last $sort (on b):

      [ {b:"x", sk:["x"]}, {b:"y", sk:["y"]}, {b:"z", sk:["z"]} ]

      Actual: the sort key is stale — it reflects the first $sort (on a):

      [ {b:"x", sk:[3]}, {b:"y", sk:[1]}, {b:"z", sk:[2]} ] 

       

      Show
        // code placeholder const c = db.getSiblingDB( "sk_bug" ).c; c.drop(); c.insertMany([ {_id: 1, a: 3, b: "x" }, {_id: 2, a: 1, b: "y" }, {_id: 3, a: 2, b: "z" }]); // Two sorts on different fields, separated by a no-op $limit so they cannot coalesce. // Final order is by 'b' (x,y,z), so {$meta: "sortKey" } should reflect 'b' . const docs = c.aggregate([ {$sort: {a: 1}}, {$limit: 1000000000}, // Any other stage would equally surface the issue. Just added this one as is a no-op, but any other else would also reproduce it {$sort: {b: 1}}, {$project: {_id: 0, b: 1, sk: {$meta: "sortKey" }}}, ]).toArray(); printjson(docs); Expected: the sort key reflects the last $sort (on b): [ {b: "x" , sk:[ "x" ]}, {b: "y" , sk:[ "y" ]}, {b: "z" , sk:[ "z" ]} ] Actual: the sort key is stale — it reflects the first $sort (on a): [ {b: "x" , sk:[3]}, {b: "y" , sk:[1]}, {b: "z" , sk:[2]} ]   
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      When an aggregation has two $sort stages on different keys that are NOT adjacent (so the optimizer cannot coalesce them), {$meta:"sortKey"} reflects the FIRST sort's key, not the effective final ordering produced by the last $sort. So it is effectively returning wrong results.

       

      A common case would be when there is a view that defines sorting and a user wants to override it with its own sorting.

       

      Note that the two sorts have to be not contiguous, which stage(s) sit in between don't matter at all

            Assignee:
            Chris Harris
            Reporter:
            Carlos Alonso Pérez
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              Created:
              Updated: