Mongodb set _id as decreasing index

Question

I want to use mongodb's default _id but in decreasing order. I want to store posts and want to get the latest posts at the start when I use find(). I am using mongoose. I tried with

postSchema.index({_id:-1})

but it didn't work

> db.posts.getIndexes()
  [
{
    "v" : 1,
    "key" : {
        "_id" : 1
    },
    "name" : "_id_",
    "ns" : "mean-dev.posts"
}]

I dropped the database and restarted mongod. No luck with that. Is there any way to set _id as a decreasing index at the sametime using mongodb's default index? I don't want to use sort() to sort the result according to _id decreasingly.

Answer 1

Short answer

You cannot a descending index on _id field. You also don't need it. Mongo will use the existing default index when doing a descending sort on the _id field.

Long answer

As stated in the documentation MongoDB index on _id field is created automatically as an ascending unique index and you can't remove it.

You also don't need to create an additional descending index on _id field because MongoDB can use the default index for sorting.

To verify that MongoDB is using index for your sorting you can use explain command:

db.coll.find().sort({_id : -1}).explain();

In the output explain command, the relevant part is

"cursor" : "BtreeCursor _id_ reverse"

which means that MongoDB is using index for sorting your query in reverse order.