MongoDB: Map/Reduce vs Aggregation framework on 1M docs. Basically just counting how many times was made each request (only 250K of requests had search queries). Aggregation turned to be 84x faster.

bm.txt

1000000 documents
249819 documents with non-empty 'search_query'

       user     system      total        real
simple map/reduce  0.270000   0.010000   0.280000 (353.701946)
simple aggregation  0.180000   0.010000   0.190000 (  8.049170)
filtering map/reduce  0.250000   0.010000   0.260000 (337.955130)
filtering aggregation  0.150000   0.010000   0.160000 (  4.095468)

mr-agg.rb

require File.expand_path('../config/environment',  __FILE__)

puts "#{RequestEvent.count} documents"
puts "#{RequestEvent.ne("data.search_query" => nil).count} documents with non-empty 'search_query'"
puts

# Simple map/reducing
def simple_map_reduce
  map = %Q{
    function() {
      emit(this.data.search_query, { count: 1 });
    }
  }

  reduce = %Q{
    function(key, values) {
      var result = { count: 1 };
      values.forEach(function(value) {
        result.count += value.count;
      });
      return result;
    }
  }

  RequestEvent.all.map_reduce(map, reduce).out(inline: 1).to_a
end

# Simple aggregation
def simple_aggregation
  pipeline = [{ "$group" => { "_id" => "$data.search_query", "count" => { "$sum" => 1 } } }]

  RequestEvent.collection.aggregate(pipeline)
end

# Map/reduce, exclude blank fields
def filtering_map_reduce
  map = %Q{
    function() {
      if (this.data.search_query)
        emit(this.data.search_query, { count: 1 });
    }
  }

  reduce = %Q{
    function(key, values) {
      var result = { count: 1 };
      values.forEach(function(value) {
        result.count += value.count;
      });
      return result;
    }
  }

  RequestEvent.all.map_reduce(map, reduce).out(inline: 1).to_a
end

# Aggregate, exclude blank fields
def filtering_aggregation
  pipeline = [
    { "$match" => { "data.search_query" => { "$ne" => nil } } },
    { "$group" => { "_id" => "$data.search_query", "count" => { "$sum" => 1 } } }
  ]

  RequestEvent.collection.aggregate(pipeline)
end

Benchmark.bm do |x|
  x.report("simple map/reduce") { simple_map_reduce }
  x.report("simple aggregation") { simple_aggregation }
  x.report("filtering map/reduce") { filtering_map_reduce }
  x.report("filtering aggregation") { filtering_aggregation }
end