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  1. Core Server
  2. SERVER-26113

explain() for aggregations pulls from the plan cache

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    • Type: Icon: Bug Bug
    • Resolution: Done
    • Priority: Icon: Major - P3 Major - P3
    • 3.5.5
    • Affects Version/s: None
    • Component/s: None
    • Fully Compatible
    • ALL
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      Run the following four commands (the last command is repeated twice):

      db.test.createIndex({x:1})
db.test.createIndex({y:1})
db.test.explain().aggregate([{$match:{x:1,y:1}}])
db.test.explain().aggregate([{$match:{x:1,y:1}}])

      Output of the second .explain() in 3.2.9 is:

      > db.test.explain().aggregate([{$match:{x:1,y:1}}])
{
	"waitedMS" : NumberLong(0),
	"stages" : [
		{
			"$cursor" : {
				"query" : {
					"x" : 1,
					"y" : 1
				},
				"queryPlanner" : {
					"plannerVersion" : 1,
					"namespace" : "test.test",
					"indexFilterSet" : false,
					"parsedQuery" : {
						"$and" : [
							{
								"x" : {
									"$eq" : 1
								}
							},
							{
								"y" : {
									"$eq" : 1
								}
							}
						]
					},
					"winningPlan" : {
						"stage" : "CACHED_PLAN",
						"inputStage" : {
							"stage" : "FETCH",
							"filter" : {
								"x" : {
									"$eq" : 1
								}
							},
							"inputStage" : {
								"stage" : "IXSCAN",
								"keyPattern" : {
									"y" : 1
								},
								"indexName" : "y_1",
								"isMultiKey" : false,
								"isUnique" : false,
								"isSparse" : false,
								"isPartial" : false,
								"indexVersion" : 1,
								"direction" : "forward",
								"indexBounds" : {
									"y" : [
										"[1.0, 1.0]"
									]
								}
							}
						}
					},
					"rejectedPlans" : [ ]
				}
			}
		}
	],
	"ok" : 1
}
      Show
      Run the following four commands (the last command is repeated twice): db.test.createIndex({x:1}) db.test.createIndex({y:1}) db.test.explain().aggregate([{$match:{x:1,y:1}}]) db.test.explain().aggregate([{$match:{x:1,y:1}}]) Output of the second .explain() in 3.2.9 is: > db.test.explain().aggregate([{$match:{x:1,y:1}}]) { "waitedMS" : NumberLong(0), "stages" : [ { "$cursor" : { "query" : { "x" : 1, "y" : 1 }, "queryPlanner" : { "plannerVersion" : 1, "namespace" : "test.test", "indexFilterSet" : false, "parsedQuery" : { "$and" : [ { "x" : { "$eq" : 1 } }, { "y" : { "$eq" : 1 } } ] }, "winningPlan" : { "stage" : "CACHED_PLAN", "inputStage" : { "stage" : "FETCH", "filter" : { "x" : { "$eq" : 1 } }, "inputStage" : { "stage" : "IXSCAN", "keyPattern" : { "y" : 1 }, "indexName" : "y_1", "isMultiKey" : false, "isUnique" : false, "isSparse" : false, "isPartial" : false, "indexVersion" : 1, "direction" : "forward", "indexBounds" : { "y" : [ "[1.0, 1.0]" ] } } } }, "rejectedPlans" : [ ] } } } ], "ok" : 1 }
    • Query 2017-03-27

      Running an explain for an aggregation pipeline appears to get its output from the plan cache. This is evident by the fact that the first stage in the winning plan is CACHED_PLAN and there are no rejected plans. This seems contrary to documented behavior. This was observed on version 3.2.9 and 3.3.8.

            Assignee:
            nicholas.zolnierz@mongodb.com Nicholas Zolnierz
            Reporter:
            christopher.harris@mongodb.com Chris Harris
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              Created:
              Updated:
              Resolved: