Index on multi-field object in mongoDB - mongodb

Question

I have a collection generated by a map reduce call, and the key is composed by two fields, so my id is set as follow :
{
"_id": {
"ts": ISODate("2014-04-22T13: 46: 00.0Z"),
"own": "LP2"
}
//... my fields
}
An index is automatically constructed on "_id" :
{
"v": NumberInt(1),
"key": {
"_id": NumberInt(1)
},
"ns": "DB.COLLECTION_NAME",
"name": "_id_"
}
But when I do my queries I often want to sort on "ts" field.
I know that I can construct an index on it, but I was wondering if the index on "_id" was already supporting it, like a compound index. That could save time on insertion.
Thanks in advance for your answers.

Answer 1

I don't believe it will. The way to test this is to insert some test data and try a few test queries. When I ran db.collectionname.find({}).sort({"_id.ts":-1}).explain() it was clear that it was not using an index for the sort:
{
"cursor" : "BasicCursor",
"isMultiKey" : false,
"n" : 13,
"nscannedObjects" : 13,
"nscanned" : 13,
"nscannedObjectsAllPlans" : 13,
"nscannedAllPlans" : 13,
"scanAndOrder" : true,
"indexOnly" : false,
"nYields" : 0,
"nChunkSkips" : 0,
"millis" : 0,
"server" : "local:27017",
"filterSet" : false
}
Note that scanandorder is true.
After adding a new index however that indexed _id.ts directly I saw better results:
db.collectionname.ensureIndex({"_id.ts":1})
db.collectionname.find({}).sort({"_id.ts":-1}).explain()
{
"cursor" : "BtreeCursor _id.ts_1 reverse",
"isMultiKey" : false,
"n" : 13,
"nscannedObjects" : 13,
"nscanned" : 13,
"nscannedObjectsAllPlans" : 13,
"nscannedAllPlans" : 13,
"scanAndOrder" : false,
"indexOnly" : false,
"nYields" : 0,
"nChunkSkips" : 0,
"millis" : 0,
"indexBounds" : {
"_id.ts" : [
[
{
"$maxElement" : 1
},
{
"$minElement" : 1
}
]
]
},
"server" : "local:27017",
"filterSet" : false
}
Note that scanandorder is now false.