that might seems a stupid question, but when should I create an index on my collection ?
To be more explicit, I was wondering if I just have to create it once, when I create my collection, and then it will be updated automatically when I add some new documents. Or do I have to regenerate it regularly in background ?
The index will be kept up-to-date by MongoDB as you update/insert documents.
Performance-wise, do not create an index until you need it (to speed up queries). And when doing massive bulk-inserts, it may be more efficient to drop the index and recreate it after you are done inserting.
MongoDB will maintain any and all indexes itself, in other words only once.
This does, however, mean you need to be careful about just what indexes you ensure as each index will create significant overhead while performing write operations. The more indexes you have the more MongoDB will have to update to do a single write.
