[{"userId":"0000","algorithm":"algo1","status":"Running","waitingTime":0},{"userId":"0001","algorithm":"algo1","status":"Received","waitingTime":0},{"userId":"0000","algorithm":"algo2","status":"Completed","waitingTime":123},{"userId":"0000","algorithm":"algo2","status":"Error","waitingTime":134},{"userId":"0001","algorithm":"algo2","status":"Error","waitingTime":150},{"userId":"0001","algorithm":"algo3","status":"Completed","waitingTime":100},{"userId":"0000","algorithm":"algo3","status":"Completed","waitingTime":120},{"userId":"0001","algorithm":"algo1","status":"Received","waitingTime":0}]

db.collection.aggregate([{"$match":{"status":"Completed"}},{$group:{"_id":"$userId","waitingTime":{"$max":"$waitingTime"}}}])