MapReduce
Contents
Introducing the MapReduce function
The MapReduce function is an aggregate function that consists of two functions: Map and Reduce.
The map is always performed before the reduce.
The map function examines every document in the collection and emits (key,value) pairs.
The map function takes no input however the current document can be accessed as this
The reduce function has two inputs, for every distinct key emitted by map the reduce function is called with a list of the corresponding values.
Population of each continent
Here the map function emits the continent and the population for each country.
The reduce function uses the JavaScript function Array.sum to add the populations.
db.world.mapReduce(
function(){emit(this.continent, this.population);},
function(k, v){ return Array.sum(v); },
{out:{inline:1}}
)
Number of countries in each continent
Instead of sending populations you can send a list one 1s to the reduce function.
That was the reduce function will count the number of countries in each continent.
db.world.mapReduce(
function(){emit(this.continent, 1);},
function(k, v){ return Array.sum(v); },
{out:{inline:1}}
)
Count only some countries
The map function may emit only sometimes.
In the example we are only counting the countries that have a large population
db.world.mapReduce(
function()
{
if (this.population>100000000)
emit(this.continent, 1);
},
function(k, v){ return Array.sum(v); },
{out:{"inline":1}}
)
Examine the reduce function
Examine the reduce function.
Here we emit the continent and the name, in the reduce function we return v.join(',') to see a comma separated list of the values in the list.
db.world.mapReduce(
function(){
if (this.population>100000000)
emit(this.continent, this.name);
},
function(k, v){ return v.join(','); },
{out:{"inline":1}}
)