costly_queries.js

result = db.log.mapReduce(
  function() { 
    if(this.hash) { 
      emit(this.hash, {
        'sum': this.duration, 
        'count': 1, 
        'sql': this.sql
      })
    }
  }, 
  function(key, values) { 
    var sum = 0; 
    var count = 0; 
    var sql = '';
    
    for(var i in values) { 
      sum += values[i].sum; 
      count += values[i].count; 
    } 
    
    return {
      'sum': sum, 
      'count': count, 
      'sql': values[0].sql
    };
    
  });
  
costs = db[result.result].find().map(function(obj) { 
  return {
    cost: obj.value.count * obj.value.sum, 
    avg: obj.value.sum / obj.value.count,
    executions: obj.value.count,
    total_time: obj.value.sum,
    sql: obj.value.sql,
    hash: obj._id
  }
}).sort(function(a, b) { 
  return a.cost - b.cost
});

print(tojson(costs));