// mongosh -f explain_bug.js let db = connect( 'mongodb: //localhost/test' ); db.adminCommand({ 'setParameter' : 1, 'internalQueryFrameworkControl' : 'forceBonsai' }); db.adminCommand({ 'configureFailPoint' : 'enableExplainInBonsai' , 'mode' : 'alwaysOn' }); const costModelOverride = { scanStartupCost: 10.5, scanIncrementalCost: 2.7, filterStartupCost: 30.3, filterIncrementalCost: 1.9 }; db.adminCommand( { 'setParameter' : 1, 'internalCostModelCoefficients' : JSON.stringify(costModelOverride)}); const documents = []; for (let i = 0; i < 1000; ++i) { documents.push({a: i, b: i % 23}) } db.testExplain.drop(); db.testExplain.insertMany(documents); const explain = db.testExplain.explain( 'executionStats' ).find({b: 10}); console.log(explain); The code above displays the following stats: PhysicalScan nodeType: 'PhysicalScan' , properties: { cost: 4640.8, localCost: 4640.8, adjustedCE: 1000, However, for physicalScan we would expect localCost = scanStartupCost + scanIncrementalCost * adjustedCE = 10.5 + 2.7*1000 = 2710.5 but got 4640.8 instead. Filter nodeType: 'Filter' , properties: { cost: 4640.8, localCost: 4640.8, adjustedCE: 1000, For filter's localCost we would expect localCost = filterStartupCost + filterIncrementalCost*adjustedCE = 30.3 + 1.9 * 1000 = 1930.3 but we got 4640.8 instead for the cost we would expect cost = filterLocalCost + scanLocalCost = 2710.5 + 1930.3 = 4640.8 and we got 4640.8 as expected. The fact that filter localCost includes PhysicalScan;'s localCost might be treated as expected behaviour, since the filter's cost includes its child's cost in the current implementaion of the CostEstimator, however, it does not look convinient.