Mongodb query execution time - mongodb

Question

I used Query below its take to much time
Query
db.saleOrder.find({"currentStatus._id":"147"},{"_id":1}).limit(10).explain("executionStats")
ExecutionStats result
{
"queryPlanner" : {
"plannerVersion" : 1,
"namespace" : "db_erp_tube.saleOrder",
"indexFilterSet" : false,
"parsedQuery" : {
"currentStatus._id" : {
"$eq" : "147"
}
},
"winningPlan" : {
"stage" : "LIMIT",
"limitAmount" : 10,
"inputStage" : {
"stage" : "PROJECTION",
"transformBy" : {
"_id" : 1
},
"inputStage" : {
"stage" : "COLLSCAN",
"filter" : {
"currentStatus._id" : {
"$eq" : "147"
}
},
"direction" : "forward"
}
}
},
"rejectedPlans" : [ ]
},
"executionStats" : {
"executionSuccess" : true,
"nReturned" : 10,
"executionTimeMillis" : 8673,
"totalKeysExamined" : 0,
"totalDocsExamined" : 3458482,
"executionStages" : {
"stage" : "LIMIT",
"nReturned" : 10,
"executionTimeMillisEstimate" : 8460,
"works" : 3458484,
"advanced" : 10,
"needTime" : 3458473,
"needYield" : 0,
"saveState" : 27019,
"restoreState" : 27019,
"isEOF" : 1,
"invalidates" : 0,
"limitAmount" : 10,
"inputStage" : {
"stage" : "PROJECTION",
"nReturned" : 10,
"executionTimeMillisEstimate" : 8450,
"works" : 3458483,
"advanced" : 10,
"needTime" : 3458473,
"needYield" : 0,
"saveState" : 27019,
"restoreState" : 27019,
"isEOF" : 0,
"invalidates" : 0,
"transformBy" : {
"_id" : 1
},
"inputStage" : {
"stage" : "COLLSCAN",
"filter" : {
"currentStatus._id" : {
"$eq" : "147"
}
},
"nReturned" : 10,
"executionTimeMillisEstimate" : 8400,
"works" : 3458483,
"advanced" : 10,
"needTime" : 3458473,
"needYield" : 0,
"saveState" : 27019,
"restoreState" : 27019,
"isEOF" : 0,
"invalidates" : 0,
"direction" : "forward",
"docsExamined" : 3458482
}
}
}
},
"serverInfo" : {
"host" : "172.16.109",
"port" : 27017,
"version" : "4.0.0",
"gitVersion" : "3b07af3d4f471ae89e8186d33bbb1d5259597d51"
},
"ok" : 1,
"operationTime" : Timestamp(1556365275, 114),
"$clusterTime" : {
"clusterTime" : Timestamp(1556365275, 114),
"signature" : {
"hash" : BinData(0,"ppu91nKmeiC//+UvdsEbjrBTDLU="),
"keyId" : NumberLong("6633468944474701825")
}
}
}

Answer 1

The query took more than 8 seconds(8673ms) to execute because it had to scan all 3458482 documents in the saleOrder collection before returning 10 documents.
This is stated in the filter stage of the explain output.
"inputStage" : {
"stage" : "COLLSCAN",
"filter" : {
"currentStatus._id" : {
"$eq" : "147"
}
},
COLLSCAN indicate full collection scan,
"totalDocsExamined" : 3458482,
This is the number of document scanned before an result is returned.
"executionTimeMillis" : 8673,
This is the total time taken for the query to run.
As #the_mahasagar suggested, build a index on currentStatus._id can speed up the query a lot, use the command below.
db.saleOrder.createIndex({ "currentStatus._id": 1})