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  1. Core Server
  2. SERVER-49469

explain executionStats for $unionWith incorrectly shows COLLSCAN for inner pipeline

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    Details

    • Backwards Compatibility:
      Minor Change
    • Operating System:
      ALL
    • Backport Requested:
      v4.4
    • Sprint:
      Query 2020-08-10, Query 2020-08-24

      Description

      db.testcoll1.createIndex({testField1:1})
      db.testcoll2.explain().aggregate( [ { $unionWith : { coll: "testcoll1", pipeline : [ { $match : { testField1 : { $gt : 2 } } } ] } } ] )
      // shows correct plan
      db.testcoll2.explain("executionStats").aggregate( [ { $unionWith : { coll: "testcoll1", pipeline : [ { $match : { testField1 : { $gt : 2 } } } ] } } ] )
      // shows incorrect plan!
      {
      	"stages" : [
      		{
      			"$cursor" : {
      				"queryPlanner" : {
      					"plannerVersion" : 1,
      					"namespace" : "test.testcoll2",
      					"indexFilterSet" : false,
      					"parsedQuery" : {
       
      					},
      					"winningPlan" : {
      						"stage" : "EOF"
      					},
      					"rejectedPlans" : [ ]
      				},
      				"executionStats" : {
      					"executionSuccess" : true,
      					"nReturned" : 0,
      					"executionTimeMillis" : 0,
      					"totalKeysExamined" : 0,
      					"totalDocsExamined" : 0,
      					"executionStages" : {
      						"stage" : "EOF",
      						"nReturned" : 0,
      						"executionTimeMillisEstimate" : 0,
      						"works" : 1,
      						"advanced" : 0,
      						"needTime" : 0,
      						"needYield" : 0,
      						"saveState" : 1,
      						"restoreState" : 1,
      						"isEOF" : 1
      					},
      					"allPlansExecution" : [ ]
      				}
      			},
      			"nReturned" : NumberLong(0),
      			"executionTimeMillisEstimate" : NumberLong(0)
      		},
      		{
      			"$unionWith" : {
      				"coll" : "testcoll1",
      				"pipeline" : [
      					{
      						"$cursor" : {
      							"queryPlanner" : {
      								"plannerVersion" : 1,
      								"namespace" : "test.testcoll1",
      								"indexFilterSet" : false,
      								"parsedQuery" : {
       
      								},
      								"queryHash" : "8B3D4AB8",
      								"planCacheKey" : "8B3D4AB8",
      								"winningPlan" : {
      									"stage" : "COLLSCAN",
      									"direction" : "forward"
      								},
      								"rejectedPlans" : [ ]
      							},
      							"executionStats" : {
      								"executionSuccess" : true,
      								"nReturned" : 0,
      								"executionTimeMillis" : 0,
      								"totalKeysExamined" : 0,
      								"totalDocsExamined" : 0,
      								"executionStages" : {
      									"stage" : "COLLSCAN",
      									"nReturned" : 0,
      									"executionTimeMillisEstimate" : 0,
      									"works" : 0,
      									"advanced" : 0,
      									"needTime" : 0,
      									"needYield" : 0,
      									"saveState" : 1,
      									"restoreState" : 0,
      									"isEOF" : 0,
      									"direction" : "forward",
      									"docsExamined" : 0
      								},
      								"allPlansExecution" : [ ]
      							}
      						},
      						"nReturned" : NumberLong(0),
      						"executionTimeMillisEstimate" : NumberLong(0)
      					}
      				]
      			},
      			"nReturned" : NumberLong(0),
      			"executionTimeMillisEstimate" : NumberLong(1)
      		}
      	],
      	"serverInfo" : {
      		"host" : "asyas-mbp-4.lan",
      		"port" : 27017,
      		"version" : "4.4.0-rc7",
      		"gitVersion" : "b79b53f55a5c148fd297b81a45c08d08e2cf8f94"
      	},
      	"ok" : 1
      }
      

      Note that index stats on testcoll1 show the index access get incremented so I believe that the error is somewhere in massaging output for executionStats for $unionWith stage. Note that the filter is missing (no parsedQuery).

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              Assignee:
              ted.tuckman Ted Tuckman
              Reporter:
              asya Asya Kamsky
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                Created:
                Updated:
                Resolved: