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  1. Core Server
  2. SERVER-57067

[SBE] Find command unexpectedly returns cursorId when final batch size aligns with result set size

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      Run the server with SBE enabled (the default in 5.0.0-rc0). Then:

      (function() {
    "use strict";
    const collname = "cursor_test";
    const coll = db.getCollection(collname);
    coll.drop();

    assert.writeOK(coll.insert({_id: 0}));
    assert.writeOK(coll.insert({_id: 1}));

    const find_response = coll.runCommand("find", {batchSize: 1});
    const cursor_id = find_response.cursor.id;                                                                                                                                                                                                         
    assert.eq(find_response.cursor.firstBatch.length, 1); 
    assert.neq(cursor_id, 0); 

    const getmore_response = coll.runCommand({getMore: cursor_id, collection: collname, batchSize: 1});
    assert.eq(getmore_response.cursor.nextBatch.length, 1); 
    assert.eq(getmore_response.cursor.id, 0, "unexpected nonzero cursor id");
}());
      Show
      Run the server with SBE enabled (the default in 5.0.0-rc0). Then: ( function () { "use strict" ; const collname = "cursor_test" ; const coll = db.getCollection(collname); coll.drop(); assert.writeOK(coll.insert({_id: 0})); assert.writeOK(coll.insert({_id: 1})); const find_response = coll.runCommand( "find" , {batchSize: 1}); const cursor_id = find_response.cursor.id; assert.eq(find_response.cursor.firstBatch.length, 1); assert.neq(cursor_id, 0); const getmore_response = coll.runCommand({getMore: cursor_id, collection: collname, batchSize: 1}); assert.eq(getmore_response.cursor.nextBatch.length, 1); assert.eq(getmore_response.cursor.id, 0, "unexpected nonzero cursor id" ); }());
    • Query Execution 2021-05-31

      In the slot-based execution engine, when running a find or getMore command with a batchSize that exactly matches the number of documents remaining to return, the command response will return a non-zero cursor id:

      > db.twodocs.drop()                                                         
true                            
> db.twodocs.insertMany([{_id: 0}, { _id: 1}])
{ "acknowledged" : true, "insertedIds" : [ 0, 1 ] }
> db.runCommand({find: "twodocs", batchSize: 1})
{
        "cursor" : {
                "firstBatch" : [
                        {
                                "_id" : 0
                        }
                ],
                "id" : NumberLong("5005687836528864628"),
                "ns" : "test.twodocs"
        },
        "ok" : 1
}
> db.runCommand({getMore: NumberLong("5005687836528864628"), collection: "twodocs", batchSize: 1})
{
        "cursor" : {
                "nextBatch" : [
                        {
                                "_id" : 1
                        }
                ],
                "id" : NumberLong("5005687836528864628"),
                "ns" : "test.twodocs"
        },
        "ok" : 1
}

      Iterating the cursor once more reveals that it is in fact exhausted. This is a regression from the classic engine, as the classic engine would have reported a cursor id of 0 to obviate the need for the final getMore where no results are returned.

      > db.runCommand({getMore: NumberLong("5005687836528864628"), collection: "twodocs", batchSize: 1})
{
        "cursor" : {
                "nextBatch" : [ ],
                "id" : NumberLong(0),
                "ns" : "test.twodocs"
        },
        "ok" : 1
}

      The problem also manifests itself with just the find command and an exact batch size matching the result set:

      > db.runCommand({find: "twodocs", batchSize: 2})
{
        "cursor" : {
                "firstBatch" : [
                        {
                                "_id" : 0
                        },
                        {
                                "_id" : 1
                        }
                ],
                "id" : NumberLong("9219549160796429098"),
                "ns" : "test.twodocs"
        },
        "ok" : 1
}
> db.runCommand({getMore: NumberLong("9219549160796429098"), collection: "twodocs"})
{
        "cursor" : {
                "nextBatch" : [ ],
                "id" : NumberLong(0),
                "ns" : "test.twodocs"
        },
        "ok" : 1
}

      Note that if the final getMore does not include a batch size, the problem doesn't manifest itself:

      > db.runCommand({find: "twodocs", batchSize: 1})
{
        "cursor" : {
                "firstBatch" : [
                        {
                                "_id" : 0
                        }
                ],
                "id" : NumberLong("1331269379857468674"),
                "ns" : "test.twodocs"
        },
        "ok" : 1
}
> db.runCommand({getMore: NumberLong("1331269379857468674"), collection: "twodocs"})
{
        "cursor" : {
                "nextBatch" : [
                        {
                                "_id" : 1
                        }
                ],
                "id" : NumberLong(0),
                "ns" : "test.twodocs"
        },
        "ok" : 1
}

            Assignee:
            nikita.lapkov@mongodb.com Nikita Lapkov (Inactive)
            Reporter:
            kyle.suarez@mongodb.com Kyle Suarez
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              Created:
              Updated:
              Resolved: