MapReduce

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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.

The reduce function will now create a count of 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 does not need to emit once for every entry.

In this 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, and 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}}
);

Reduce to a single value

If you emit the same key every time you will get exactly one result from your query.

Here we emit the value 1 as the key and 1 as the value. The reduce function sums those 1s to get a count of the total number of countries.

db.world.mapReduce(
  function () {
    emit(1, 1);
  },
  function (k, v) { return Array.sum(v); },
  {out: {"inline": 1}}
);

Emit a name

You can use the list given in the reduce function.

Here we emit the key this.continent and the value this.name. The reduce function returns the first element of the collected list.

db.world.mapReduce(
  function () {
    emit(this.continent, this.name);
  },
  function (k, v) { return v[0]; },
  {out: {"inline": 1}}
);

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