indexing - MongoDB compound index usage

Question

Lets say I have document with the following two keys:

1) key1
2) key2

If I am creating compound index on both of them..

{'key1':1,'key2':1}

When running a query relevant only for key1.. does the index above is used? or I need to create specific index only for key1 also?

Thanks

Answer 1

In MongoDB, you can use index prefix to query the database. You can't use anything else. If your query does not contain key prefix the index won't be used.

Assuming your proposed index {'key1':1,'key2':1}:

Queries that will use index:

• db.some.find({key1 : {$gt : 100}}) - uses prefix
• db.some.find({key1 : {$gt : 100}, key2 : {$lt : 30}}) - uses full index
• db.some.find({key3 : 'test'}).sort({key1 : 1}) - uses prefix for sort (direction match)

Queries that will NOT use index:

• db.some.find({key2 : {$gt : 100}}) - index order matters - key2 is not prefix
• db.some.find({key3 : 'test'}).sort({key1 : -1}) - index direction matters for multicolumn indexes
• db.some.find({key3 : 'test'}).sort({key2 : 1}) - it's not prefix