mongodb $near query is slow - mongodb

Question

One mongodb collection
{
"_id" : ObjectId("574bbae4d009b5364abaebe5"),
"cityid" : 406,
"location" : {
"type" : "Point",
"coordinates" : [
118.602355,
24.89083
]
},
"shopid" : "a"
}
with about 50, 000 rows;
and indexes:
[
{
"v" : 1,
"key" : {
"_id" : 1
},
"name" : "_id_",
"ns" : "pingan-test.shop_actinfo_collection_0530"
},
{
"v" : 1,
"key" : {
"location" : "2dsphere"
},
"name" : "location_2dsphere",
"ns" : "pingan-test.shop_actinfo_collection_0530",
"2dsphereIndexVersion" : 3
},
{
"v" : 1,
"key" : {
"shopid" : 1,
"cityid" : 1
},
"name" : "shopid_1_cityid_1",
"ns" : "pingan-test.shop_actinfo_collection_0530"
}
]
I query this collection like:
body = {'cityid': 2, 'location': {'$near': {'$geometry': {'type': 'Point', 'coordinates': [122.0, 31.0]}}}, 'shopid': {'$in': ['a','b']}}
results = collection.find(body, {'shopid': 1, '_id':0},).batch_size(20).limit(20)
shops = list(results)
The question is that it run about 400ms. But it just take 30ms if we don't care about location.
why and how to fix? please.

Answer 1

You have an index on shopid and cityid, but you search for cityid. Since the index is ordered by shopid first it cannot be used to search by cityid. If you change the index to cityid: 1, shopid: 1, then you will see a performance improvement because your query will be able to search using the index.

Answer 2

after all, i got it.
I just create a index to cityid: 1, shopid: 1, "location" : "2dsphere"
, and then, world peace。
and thanks #tiramisu again.