Queries in Realtime-database (using LimitToLast) are very very slow - firebase

Question

I'm using RealTime-database(Firebase 7.3.2) and Unity.
When I'm using the LimitToLast() method the query takes a long time(1,5 to 2 minutes) to return a reponse.
But when I load the whole data or execute this query without the LimitToLast method this takes not a long time.
I want to ask if everyone has this problem during his development with realtime firebase database.
My database contains 1700 rooms.
this is the query :
var result = await FirebaseDatabase.DefaultInstance.GetReference("Rooms")
.OrderByChild("CreationDate").LimitToLast(10).GetValueAsync();
And that is the structur of rooms collection in database:
{
"Rooms" : {
"-Lp860kFH8TjdAsPpar1" : {
"CreationDate" : -14400,
"Title" : "Room 1",
...,
},
"-Lp860kFH8TjdAsPpbr2" : {
"CreationDate" : -14402,
"Title" : "Room 2",
...,
},
...
"-Lp860kFH8TjdAsPpar3" : {
"CreationDate" : -14404,
"Title" : "Room 1700",
...,
}
}
}

Answer 1

Are you sure you have indexing done in your Firebase Realtime Database Security Rules? If its not done, then the query is executed as follows:
1. Download all the data from the "Rooms" branch to the Unity client.
2. Sort the data according to your ordering criteria on the Unity client.
3. Discard all except the last 10 children in this sorted data.
I'm sure nobody would want to do that if you want to get just the last 10 children. The ordering and limiting to last 10 children should happen on the database server itself
which will ensure it to be fast enough to give you the result in milliseconds. For that, you'll have to index your data and then run your queries.