MongoDB - Unique index vs compound index

Question

Assume a hypothetical document with 3 fields:

1. _id : ObjectId
2. emailAddress : string
3. account : string

Now, given a query on emai

Answer 1

When it comes to Indexes, the goal is to create a single index with highest possible cardinality (or "selectivity"). Try to write queries that use 1 (compounded) index per query. Unique indexes have maximum cardinality. Compounding unique indexes with less selective fields can not further increase that maximum. Adding more indexes just slows down find(), update() and remove() queries. So be "lean and mean".

However, if you are using sort() on the account field, while doing a find() on the email field, then you should use a compound index:

it's common to query on multiple keys and to sort the results. For these situations, compound indexes are best. http://www.mongodb.org/display/DOCS/Indexing+Advice+and+FAQ

So think it through! If you need to sort data by another field, then you usually need a compound index.