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  1. Core Server
  2. SERVER-35973

Explain execution stats for $sample are not reported correctly

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    • Backwards Compatibility:
      Fully Compatible
    • Operating System:
      ALL
    • Sprint:
      Query 2018-07-16, Query 2018-07-30

      Description

      The [MULTI_ITERATOR] stage used as part of $sample execution reports all zeros in place of real execution stats:

      MongoDB Enterprise > db.c.drop()
      false
      MongoDB Enterprise > for (let i = 0; i < 1000; i++) { db.c.insert({_id: i}); }
      WriteResult({ "nInserted" : 1 })
      MongoDB Enterprise > db.c.explain(true).aggregate([{$sample: {size: 10}}])
      {
      	"stages" : [
      		{
      			"$cursor" : {
      				"query" : {
       
      				},
      				"queryPlanner" : {
      					"plannerVersion" : 1,
      					"namespace" : "test.c",
      					"indexFilterSet" : false,
      					"winningPlan" : {
      						"stage" : "MULTI_ITERATOR"
      					},
      					"rejectedPlans" : [ ]
      				},
      				"executionStats" : {
      					"executionSuccess" : true,
      					"nReturned" : 0,
      					"executionTimeMillis" : 43,
      					"totalKeysExamined" : 0,
      					"totalDocsExamined" : 0,
      					"executionStages" : {
      						"stage" : "MULTI_ITERATOR",
      						"nReturned" : 0,
      						"executionTimeMillisEstimate" : 0,
      						"works" : 0,
      						"advanced" : 0,
      						"needTime" : 0,
      						"needYield" : 0,
      						"saveState" : 0,
      						"restoreState" : 0,
      						"isEOF" : 0,
      						"invalidates" : 0
      					},
      					"allPlansExecution" : [ ]
      				}
      			}
      		},
      		{
      			"$sampleFromRandomCursor" : {
      				"size" : NumberLong(10)
      			}
      		}
      	],
      	"ok" : 1
      }
      

      Notice that the counts ("works", "advanced", "nReturned", etc.) associated with MULTI_ITERATOR in the "executionStats" section are all zeros. This is a reporting issue in the server; the query clearly returns results when run without explain:

      MongoDB Enterprise > db.c.aggregate([{$sample: {size: 10}}])
      { "_id" : 266 }
      { "_id" : 789 }
      { "_id" : 999 }
      { "_id" : 987 }
      { "_id" : 685 }
      { "_id" : 979 }
      { "_id" : 633 }
      { "_id" : 484 }
      { "_id" : 847 }
      { "_id" : 22 }
      

      It looks like the issue is that MultiIterator::getStats() is not implemented correctly.

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              Assignee:
              david.storch David Storch
              Reporter:
              david.storch David Storch
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                Dates

                Created:
                Updated:
                Resolved: