The document discusses MongoDB aggregation techniques including count, distinct, group, and mapReduce. It provides examples of how to use each technique including counting documents, finding distinct values, grouping documents by a field, and using mapReduce to parallelize operations. MapReduce jobs in MongoDB are currently single threaded but sharding or external parallelization can be used to improve performance.
16. MapReduce Parallelism As of right now, MapReduce jobs on a single mongod process are single threaded. This is due to a design limitationin current JavaScript engines. We are looking into alternatives to solve this issue, but for now if you want to parallelize your MapReduce jobs, you will need to either use shardingor do the aggregation client-side in your code.