MongoDB: Map-Reduce-mysql tutorial

Map-reduce is a data processing program (paradigm) that considers large data to obtain useful aggregation results ). for map-reduce operations, MongoDB provides mapreduce commands. consider the following map-reduce operation: in MongoDB, the map-reduce Operation uses a custom function to map or associate keys. If

Map-reduce is a data processing program (paradigm) that considers large data to obtain useful aggregation results ). for map-reduce operations, MongoDB provides mapreduce commands. consider the following map-reduce operation: in MongoDB, the map-reduce Operation uses a custom function to map or associate keys. If

Map-reduce is a data processing program (paradigm) that considers large data to obtain useful aggregation results. for map-reduce operations, MongoDB provides mapreduce commands.

Consider the following map-reduce operation:

JavaScript functions

In MongoDB, map-reduce operations use custom functions to map, or associate keys and values. If a key has multiple values corresponding to it, the reduce operation "removes" the value of the key to a single object (the operation causes CES the values for the key to a single object ).

Custom javascript functions provide flexibility for map-reduce. For example, when processing a document, the map function generates more than one key-value pair matching or no key-value pair matching. The Map-reduce function can also use a custom javascript function to modify the result at the end of map and reduce functions.

Map-Reduce action

In MongoDB, the map-reduce function can write results or return results to the set online. If you write the map-reduce output to a set, you can execute subsequent map-reduce operations on the same input set, and merge and replace the operations, or cut the previous results.

When the result of the map-reduce operation is returned online, the result Document must be within the BSON Document Size limit, which is currently 16 MB.

MongoDB supports map-reduce operations on shared sets and outputs results to shared sets.