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

Answer 2

Yes. In a B-tree index, you can use a prefix of the columns.

So you can use the index for a query on 'key1' (but not as efficiently for 'key2', the column order in the index matters).

This is the same situation as in a printed telephone book, which is an index on [lastName, firstName]. You can use that to look up people by lastName easily (and not so easily by firstName, but still more efficient than calling everyone and asking for their first name).