Question

Point out the wrong statement.

(a) If the output collection does not exist, MongoDB creates and shards the collection on the _id field

(b) Map-reduce supports operations on sharded collections, both as an input and as an output

(c) mongos dispatches, in parallel, a map-reduce post-processing job to every shard that owns a chunk

(d) None of the mentioned

The question was asked in an online quiz.

I'm obligated to ask this question of Map-Reduce and Sharded Collections in portion Aggregation Concepts and Aggregation Mechanics of MongoDB

Answer 1

Right answer is (d) None of the mentioned

The explanation: When using sharded collection as the input for a map-reduce operation, mongos will automatically dispatch the map-reduce job to each shard in parallel.