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Index Your Data

Firebase allows you to do ad-hoc queries on your data using an arbitrary child key. If you know in advance what your indexes will be, you can define them via the .indexOn rule in your Firebase Realtime Database Rules to improve query performance.

Defining Data Indexes

Firebase provides powerful tools for ordering and querying your data. Specifically, Firebase allows you to do ad-hoc queries on a collection of nodes using any common child key. As your app grows, the performance of this query degrades. However, if you tell Firebase about the keys you will be querying, Firebase will index those keys at the servers, improving the performance of your queries.

Indexing with orderByChild

The easiest way to explain this is through an example. All of us at Firebase agree that dinosaurs are pretty cool. Here's a snippet from a sample database of dinosaur facts. We will use it to explain how .indexOn works with orderByChild().

{
  "lambeosaurus": {
    "height" : 2.1,
    "length" : 12.5,
    "weight": 5000
  },
  "stegosaurus": {
    "height" : 4,
    "length" : 9,
    "weight" : 2500
  }
}

Let's imagine that in our app, we often need to order the dinosaurs by name, height, and length, but never by weight. We can improve the performance of our queries by telling Firebase this information. Since the name of the dinosaurs are just the keys, Firebase already optimizes for queries by dinosaur name, since this is the key of the record. We can use .indexOn to tell Firebase to optimize queries for height and length as well:

{
  "rules": {
    "dinosaurs": {
      ".indexOn": ["height", "length"]
    }
  }
}

Like other rules, you can specify an .indexOn rule at any level in your rules. We placed it at the root level for the example above because all the dinosaur data is stored at the root of the database.

Indexing with orderByValue

In this example, we'll demonstrate how .indexOn works with orderByValue(). Let's say we're making a leaderboard of dino sports scores with the following data:

{
  "scores": {
    "bruhathkayosaurus" : 55,
    "lambeosaurus" : 21,
    "linhenykus" : 80,
    "pterodactyl" : 93,
    "stegosaurus" : 5,
    "triceratops" : 22
  }
}

Since we're using orderByValue() to create the leaderboard, we can optimize our queries by adding a .value rule at our /scores node:

{
  "rules": {
    "scores": {
      ".indexOn": ".value"
    }
  }
}