Why does this firestore query require an index?

Why does this Firestore query require an index?

As you probably noticed, queries in Cloud Firestore are very fast and this is because Firestore automatically creates an index for any field you have in your document. So when you simply filter with a range comparison, Firestore creates the required index automatically. If you also try to order your results, another index is required. This kind of index is not created automatically. You should create it yourself. This can be done, by creating it manually in your Firebase Console or you'll find in your logs a message that sounds like this:

FAILED_PRECONDITION: The query requires an index. You can create it here: ...

You can simply click on that link or copy and paste the URL into a web browser and your index will be created automatically.

So Firestore requires an index so you can have very fast queries.

An index is simply a database inventory or a record of what is where. And each index is a specific inventory of a specific thing—for example, how many propertyX fields exist in a collection and what their values are, sorted (the fact that they are sorted is critical).

If this inventory didn't exist, to query for documents where propertyX is someValue, the machine would have to iterate over the entire collection to determine (1) which documents contain propertyX and (2) which documents contain propertyX equal to someValue. By keeping an inventory (or index) of queried properties, when a query is performed on propertyX, the machine can go straight to the propertyX index and gather the locations of all the documents that equal someValue and then fetch those documents from the collection and return them. Not only does the machine not need to touch the collection to know where the documents are but it doesn't even need to iterate over the entire index because it's always in order.

Indexes are why collection sizes have no impact on the performance of Firestore queries and why we only need to index properties that are ever queried.