Question

In MongoDB I have a query which looks like this to find out for which comments the user has already voted:

db.comments.find({
    _id: { $in: [...some ids...] },
    votes.uid: "4fe1d64d85d4f4c00d000002"
});

As the documentation says you should have

One index per query

So what’s better creating a multikey on _id + votes.uid or is it enough to just index on votes.uid because Mongo handles _id automatically in any way?

Answer 1

There is automatically an index on _id.

Depending of your queries (how many ids you have in the $in array) and your data, (how many votes you have on one object) you may create a index on votes.uid.

Take care of which index is used during query execution and remember you can force Mongo to use the index you want by adding .hints(field:1) or hints('indexname')