Starts mongod with --setParameter enableTestCommands=1 // Inserts any decimal value db.opt.insert({n: NumberDecimal( "0" )}) // Turns off pipeline optimization db.adminCommand({configureFailPoint: "disablePipelineOptimization" , mode: "alwaysOn" }); // In mixed type $add, if there's any decimal input ($n), the final result is promoted to decimal. // While doing addition, the double double sum algorithm accumulates non-decimal values into a separate nonDecimalTotal state. // So, we don't lose precision here. // Note that int64 can't be exactly expressed. db.opt.aggregate({$project: {_id: 0, o: {$add: [ "$n" , 0, NumberLong( "111111111111111111" )]}}}); { "o" : NumberDecimal( "111111111111111111" ) } // Enables pipeline optimization db.adminCommand({configureFailPoint: "disablePipelineOptimization" , mode: "off" }); // Here an interesting thing happens when a pipeline is optimized. This is the SBE result. // While optimizing the pipeline, double 0 and NumberLong( "111111111111111111" ) is optimized into a single double value. // And the SBE binary $add algorithm promotes the double to a decimal with 15 digit precision and so we lose 3 digit precision here. // This issue happens because the optimization passes the final result instead of the intermediate results in the form of // decimalTotal and nonDecimalTotal. db.adminCommand({getParameter: 1, internalQueryForceClassicEngine: 1}); { "internalQueryForceClassicEngine" : false , "ok" : 1 } db.opt.aggregate({$project: {_id: 0, o: {$add: [ "$n" , 0, NumberLong( "111111111111111111" )]}}}); { "o" : NumberDecimal( "111111111111111000" ) } // This is the classic engine's result. // The classic engine's double double sum algorithm promotes the optimization result double value to a decimal with 34 digit // precision. So, we keep the double value as close as possible to the addition result which is 111111111111111104.0. // But the real issue here is that the optimization passes the final result instead of the intermediate results in the form of // decimalTotal and nonDecimalTotal. db.adminCommand({setParameter: 1, internalQueryForceClassicEngine: true }); { "was" : false , "ok" : 1 } db.opt.aggregate({$project: {_id: 0, o: {$add: [ "$n" , 0, NumberLong( "111111111111111111" )]}}}); { "o" : NumberDecimal( "111111111111111104" ) }