mongoDB MapReduce文档

MapReduce

Map/reduce in MongoDB is useful for batch processing of data and aggregation operations. It is similar in spirit to using something like Hadoop with all input coming from a collection and output going to a collection. Often, in a situation where you would have used GROUP BY in SQL, map/reduce is the right tool in MongoDB.

• Overview
  • Incremental Map-reduce
  • Output options
  • Result object
  • Map Function
  • Reduce Function
    • A more technical explanation
  • Finalize Function
  • jsMode flag
• Sharded Environments
  • Sharded input
  • Sharded output
• Examples
  • Shell Example 1
  • Shell Example 2
  • Mongo Shell Script with Incremental Map-Reduce and Finalize
  • More Examples
  • Note on Permanent Collections
• Parallelism
• Presentations
• Troubleshooting
• See Also

Related driver docs: Python , Ruby , Java , PHP , Perl , C++