MongoDB Map Reduce

Map-reduce is a computational model, which simply means that a large amount of work (data) decomposition (MAP) is performed, and then the results are combined into the final result (Reduce).

Basic syntax for the MapReduce command:

Db.collection.mapReduce (

function () {emit (Key,value);},//map functions

function (key,values) {return reducefunction},//reduce functions

{

Out:collection,

Query:document,

Sort:document,

Limit:number

}

)

UseMapReduceto implement two functionsMapfunctions andReducefunction, Mapfunction Callemit (key, value),TraverseCollectionall the records in, will beKeywith thevaluepassed toReducefunction for processing.

The Map function must call emit (key, value) to return a key-value pair.

Parameter Description :

Map: Map function ( generate key-value pair sequence as the reduce function parameter ) .

Reduce: Statistical functions, The task of the reduce function is to turn key-values into key-value, which means The values array becomes a single value .

Out: The statistical result holds the collection ( without specifying the use of a temporary collection , which is automatically deleted after the client disconnects ).

Query: A filter condition in which only documents that meet the criteria call the map function. (Query,limit,sort can be combined arbitrarily)

Sort: sort parameter combined with limit (also in the Send to map To sort the document before the function), you can optimize the grouping mechanism.

Limit: The upper limit of the number of documents sent to the map function (if no limitisused, using sort alone is not very useful)

Examples of Use:

Db.posts.mapReduce (

function () {emit (this.user_name,1);},

function (key, values) {return array.sum (values)},

{

Query:{status: "Active"},

Out: "Post_total"

}

). Find ()

MongoDB Map Reduce