Firebase Complex Query orderByChild() working slow

firebase_complex_query

Problem:
Slow response on complex query method orderByChild("key")

Inspection:
Check the websocket frames on Chrome DevTools.
If it is loading unnecessary data, it means that Firebase is doing the query on client side. (Yes, it's not in the documentation but it does that)

Solution:
Add indexing in Security rules

Example:
DB: 
{
  "collection": {
    "item_id": {
      "key": 1,
      "another_key": "veg"
    }
  }
}

Security Rules:
"collection": {
  ".indexOn": ["key", "another_key"]
}