Firestore checking if username exists, best model to not query the whole database?

Question

Hello I'm developing and Android app and using Firebase's Firestore. My concern is about creating a username for my user when he is signing up for my app. I know I have to check if the username exists in my database, but what if you have 1 million users or 5. I don't think the results will be fast when you query the whole database. Is querying the whole database the only approach? or maybe creating a collection called usernames with 24 documents inside and for example the first document holds collection of usernames starting with a, then second document holds collection of usernames starting with b, and so on. Need your help. Thank you.

Answer 1

Actually one of the key characteristics of Firestore is exactly that: the performance of a query is proportional to the size of your result set, not your data set.

So the query performance that you get for finding 1 document in a collection of 5, 24 or 1 million docments will be exactly the same.

In Cloud Firestore, you can use queries to retrieve individual, specific documents or to retrieve all the documents in a collection that match your query parameters. Your queries can include multiple, chained filters and combine filtering and sorting. They're also indexed by default, so query performance is proportional to the size of your result set, not your data set.

So the answer is that you should query your already existing collection of documents and not create smaller collection(s) with a subset of documents for the sake of query performance.