Indexing with orderByChild

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

{  "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, andlength, but never by weight. We can improve the performance of our queries by telling Firebasethis information. Since the name of the dinosaurs are just the keys, Firebase alreadyoptimizes 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 storedat the root of the database.

Indexing with orderByValue

In this example, we'll demonstrate how.indexOn works withorderByValue().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"    }  }}

Indexes are not required for development unless you are using the REST API. The realtime client libraries can execute ad-hoc queries without specifying indexes. Performance will degrade as the data you query grows, so it is important to add indexes before you launch your app if you anticipate querying a large set of data.