How does Mongodb $pull work under the hood - mongodb

Question

Imagine the following two documents in a myCollection:
{_id: '1', fruits: ['Apple', 'Banana', 'Lemon']}
{_id: '1', fruits: ['Apple', 'Lemon']}
Is there a difference of performance between:
db.myCollection.update({}, {$pull: 'Banana'}, {multi: true)}
and:
db.myCollection.update({fruits: {$elemMatch: 'Banana'}}, {$pull: 'Banana'}, {multi: true)}

Answer 1

//output from mongo client
//ran below queries and compared the output from explain. I did not find any difference in //execution stats for performance
> db.myCollection.explain("executionStats").update(
... {},
... {$pull:{fruits:{$in:["Banana"]}}},
... {multi: true}
... );
{
"queryPlanner" : {
"plannerVersion" : 1,
"namespace" : "test.myCollection",
"indexFilterSet" : false,
"parsedQuery" : {
},
"winningPlan" : {
"stage" : "UPDATE",
"inputStage" : {
"stage" : "COLLSCAN",
"direction" : "forward"
}
},
"rejectedPlans" : [ ]
},
"executionStats" : {
"executionSuccess" : true,
"nReturned" : 0,
"executionTimeMillis" : 0,
"totalKeysExamined" : 0,
"totalDocsExamined" : 2,
"executionStages" : {
"stage" : "UPDATE",
"nReturned" : 0,
"executionTimeMillisEstimate" : 0,
"works" : 4,
"advanced" : 0,
"needTime" : 3,
"needYield" : 0,
"saveState" : 0,
"restoreState" : 0,
"isEOF" : 1,
"nMatched" : 2,
"nWouldModify" : 2,
"wouldInsert" : false,
"inputStage" : {
"stage" : "COLLSCAN",
"nReturned" : 2,
"executionTimeMillisEstimate" : 0,
"works" : 4,
"advanced" : 2,
"needTime" : 1,
"needYield" : 0,
"saveState" : 2,
"restoreState" : 2,
"isEOF" : 1,
"direction" : "forward",
"docsExamined" : 2
}
}
},
"serverInfo" : {
"host" : "XXXX",
"port" : XXXX,
"version" : "4.2.6",
"gitVersion" : "XXXX"
},
"ok" : 1
}
>
--
> db.myCollection.explain("executionStats").update(
... {},
... {$pull:
... {fruits:{$elemMatch:{$in:["Banana"]}}}
... },
... {multi: true}
... );
{
"queryPlanner" : {
"plannerVersion" : 1,
"namespace" : "test.myCollection",
"indexFilterSet" : false,
"parsedQuery" : {
},
"winningPlan" : {
"stage" : "UPDATE",
"inputStage" : {
"stage" : "COLLSCAN",
"direction" : "forward"
}
},
"rejectedPlans" : [ ]
},
"executionStats" : {
"executionSuccess" : true,
"nReturned" : 0,
"executionTimeMillis" : 0,
"totalKeysExamined" : 0,
"totalDocsExamined" : 2,
"executionStages" : {
"stage" : "UPDATE",
"nReturned" : 0,
"executionTimeMillisEstimate" : 0,
"works" : 4,
"advanced" : 0,
"needTime" : 3,
"needYield" : 0,
"saveState" : 0,
"restoreState" : 0,
"isEOF" : 1,
"nMatched" : 2,
"nWouldModify" : 2,
"wouldInsert" : false,
"inputStage" : {
"stage" : "COLLSCAN",
"nReturned" : 2,
"executionTimeMillisEstimate" : 0,
"works" : 4,
"advanced" : 2,
"needTime" : 1,
"needYield" : 0,
"saveState" : 2,
"restoreState" : 2,
"isEOF" : 1,
"direction" : "forward",
"docsExamined" : 2
}
}
},
"serverInfo" : {
"host" : "XXXX",
"port" : XXX,
"version" : "4.2.6",
"gitVersion" : "XXXX"
},
"ok" : 1
}
>