Firestore's where query - number of reads charged

Question

This may sound silly, but I needed to rule out the possibilities behind the document reads of my app on Firestore. When the where query is used using client SDK, are all the documents downloaded to the client and then filtered or they are filtered at the server and returned? Also, what is the case when we use where query with cloud functions?

E.g.: Let's say collection posts have 1000 documents and 10 matches the criteria mentioned in the where query, how many reads I would get charged for if I use

1. Client SDK 2) Cloud Functions

Answer 1

When the where query is used using client SDK, are all the documents downloaded to the client and then filtered or they are filtered at the server and returned?

No, the documents are filtered on the server. You'll only get the documents that represent the result of the query.

Also, what is the case when we use where query with cloud functions?

That's the exact same thing as above.

Let's say collection posts have 1000 documents and 10 match the criteria mentioned in the where query, how many reads I would get charged for if I use

Only 10. You'll always be billed for the number of documents that you read. For instance, if your query returns only ten documents, you be billed only with ten document reads, and nothing more. Bear in mind that it doesn't matter the number of documents you are searching in, but the number of documents your query returns.