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  1. Core Server
  2. SERVER-31406

Running aggregate cmd with explain:false is the same as cursor:{}

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    • Query Optimization
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      In the MongoDB shell:

      test> db.version()
3.5.12-380-g168f195001
test> db.test.insert([{x:1},{x:2},{x:3}]);

      Try aggregating without the cursor document:

      test> db.runCommand({aggregate: "test", pipeline: []})
{
	"operationTime" : Timestamp(1507216222, 1),
	"ok" : 0,
	"errmsg" : "The 'cursor' option is required, except for aggregate with the explain argument",
	"code" : 9,
	"codeName" : "FailedToParse"
}

      Now with the cursor document:

      test> db.runCommand({aggregate: "test", pipeline: [], cursor:{}})
{
	"cursor" : {
		"firstBatch" : [
			{
				"_id" : ObjectId("59d64b3c374bbed61ea8ba4a"),
				"x" : 1
			},
			{
				"_id" : ObjectId("59d64b3c374bbed61ea8ba4b"),
				"x" : 2
			},
			{
				"_id" : ObjectId("59d64b3c374bbed61ea8ba4c"),
				"x" : 3
			}
		],
		"id" : NumberLong(0),
		"ns" : "test.test"
	},
	"ok" : 1,
	"operationTime" : Timestamp(1507216252, 1)
}

      But it behaves the same as passing explain:false

      test> db.runCommand({aggregate: "test", pipeline: [], explain:false})
{
	"cursor" : {
		"firstBatch" : [
			{
				"_id" : ObjectId("59d64b3c374bbed61ea8ba4a"),
				"x" : 1
			},
			{
				"_id" : ObjectId("59d64b3c374bbed61ea8ba4b"),
				"x" : 2
			},
			{
				"_id" : ObjectId("59d64b3c374bbed61ea8ba4c"),
				"x" : 3
			}
		],
		"id" : NumberLong(0),
		"ns" : "test.test"
	},
	"ok" : 1,
	"operationTime" : Timestamp(1507216292, 1)
}
      Show
      In the MongoDB shell: test> db.version() 3.5.12-380-g168f195001 test> db.test.insert([{x:1},{x:2},{x:3}]); Try aggregating without the cursor document: test> db.runCommand({aggregate: "test" , pipeline: []}) { "operationTime" : Timestamp(1507216222, 1), "ok" : 0, "errmsg" : "The 'cursor' option is required, except for aggregate with the explain argument" , "code" : 9, "codeName" : "FailedToParse" } Now with the cursor document: test> db.runCommand({aggregate: "test" , pipeline: [], cursor:{}}) { "cursor" : { "firstBatch" : [ { "_id" : ObjectId( "59d64b3c374bbed61ea8ba4a" ), "x" : 1 }, { "_id" : ObjectId( "59d64b3c374bbed61ea8ba4b" ), "x" : 2 }, { "_id" : ObjectId( "59d64b3c374bbed61ea8ba4c" ), "x" : 3 } ], "id" : NumberLong(0), "ns" : "test.test" }, "ok" : 1, "operationTime" : Timestamp(1507216252, 1) } But it behaves the same as passing explain:false test> db.runCommand({aggregate: "test" , pipeline: [], explain: false }) { "cursor" : { "firstBatch" : [ { "_id" : ObjectId( "59d64b3c374bbed61ea8ba4a" ), "x" : 1 }, { "_id" : ObjectId( "59d64b3c374bbed61ea8ba4b" ), "x" : 2 }, { "_id" : ObjectId( "59d64b3c374bbed61ea8ba4c" ), "x" : 3 } ], "id" : NumberLong(0), "ns" : "test.test" }, "ok" : 1, "operationTime" : Timestamp(1507216292, 1) }

      When running aggregate on MongoDB 3.5.12, it seems like the option explain:false has the same effect as passing an empty cursor document. Per SERVER-24623 the command should be required to pass explain:true or an empty cursor document.

            Assignee:
            backlog-query-optimization [DO NOT USE] Backlog - Query Optimization
            Reporter:
            kevin.albertson@mongodb.com Kevin Albertson
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              Created:
              Updated: