Slow mongodb query, how can it happen? - mongodb

Question

I have the following query in MongoDB:
db.getCollection('message').aggregate([
{
"$match": {
"who" : { "$in" : ["manager", "woker"] },
"sendTo": { "$in": ["userId:243369", "userId:160921"] },
"exceptSendTo": { "$nin": ["userId:37355"] },
"msgTime": { "$lt": 1559716155 },
"isInvalid": { "$exists": false }
}
},
{
"$sort": { "msgTime": 1, "who": 1, "sendTo": 1 }
},
{
"$group": { "_id": "$who", "doc": { "$first": "$type" } }
}
], { allowDiskUse: true})
forget about the field meaning. and I have this index:
/* 1 */
[
{
"v" : 1,
"key" : {
"_id" : 1
},
"name" : "_id_",
"ns" : "db.message"
},
{
"v" : 1,
"key" : {
"who" : 1.0,
"sendTo" : 1.0
},
"name" : "who_sendTo",
"ns" : "db.message"
},
{
"v" : 1,
"key" : {
"msgTime" : 1.0
},
"name" : "msgTime_1",
"ns" : "db.message"
},
{
"v" : 1,
"key" : {
"msgTime" : 1.0,
"who" : 1.0,
"sendTo" : 1.0
},
"name" : "msgTime_1.0_who_1.0_sendTo_1.0",
"ns" : "db.message",
"background" : true
}
]
Perform the query above, It cost 1.52s, use explain to see it indeed has used msgTime_1.0_who_1.0_sendTo_1.0 index.
Why is query is still low while index has been used? and is there any way to solve the low problem like change index or something?

Answer 1

I dont think you are using the sort at all the way you intend to use it.
The $firs argument requires a sort on the actual first arguement
https://docs.mongodb.com/manual/reference/operator/aggregation/first/
You need to sort the key you want the first element of.
OR you could use $$ROOT, witch returns the first document.
I think you should modify it to something like:
{"$sort": {"who": 1, "msgTime": 1, "sendTo": 1}},
{"$group": {"_id": "$who", "doc": {"$first": "$$root"}}},
In this case the $group operator can find the result for each group "instantly" since they are all next to each other.
If you are only interested in the type, add an projection:
{'$project': {'doc.type': 1}