How do you read a part of key-value data without loading the whole datastore

Cross posted here: http://stackoverflow.com/questions/32173849/how-do-you-read-a-part-of-key-value-data-without-loading-the-whole-datastore

Take for example from the ds = datastore('mapreduce.out') datastore from this example

If you did a readall(ds) you would retrieve a list of all keys and values, which looks like:

Key        Value 
________    _______

'AA'        [14930]
'AS'        [ 2910]
'CO'        [ 8138]
'DL'        [16578]
'EA'        [  920]
'HP'        [ 3660]
'ML (1)'    [   69]
'NW'        [10349]
.            .
.            .
.            .

What if you only wanted data from the Keys 'AA', 'DL' and 'NW'? Do you still have to read all the data in the datastore at once?

How can you find a list of keys in the datasource and only read data corresponding to these keys?