Before diving into schema design, it's super important to get a good grasp of how Firebase Realtime Database is set up. It's a NoSQL database, which means data is stored as JSON and synced in real-time to every connected client. Pretty cool, right?

Figure out what data you need to store. Depending on what your app does, this could be user profiles, user-generated content, analytics data, and so on. Break this data down into categories.

Start with a top-level root object and define the main categories you identified earlier as properties of this object. Each of these properties can be an object itself, holding related data as its properties.

The main goal here is to cut down on redundancy and avoid having huge objects that need to be downloaded just to get small bits of data. Also, keep Firebase's constraints in mind, like the maximum depth of a child path being 32 levels.

While Firebase automatically indexes data by key, you might need to create extra indexes for other queries your app will use. This is especially important if your app needs to filter or order data by a specific child key. To create an index, use the ".indexOn" rule in your Firebase Realtime Database Security Rules.

Just like with relational databases, data normalization is key in Firebase to avoid redundancies and inconsistencies. Since Firebase is a NoSQL database, data duplication is more acceptable than in SQL, as joining data from different paths isn't an option. But too much duplication can lead to sync issues and data inconsistencies. Aim to find the right balance, denormalizing data when needed for performance reasons.

Firebase doesn't support server-side joins, but you can do this through client-side processing. For example, if a blog post object only has the author's ID, you can use this ID to get the author’s detailed info from the author’s path in the database. In your app, fetch the blog post first, then fetch the corresponding author info based on the ID.

Firebase Realtime Database offers strong client-side security through Firebase security rules. These rules decide who can read and write to which parts of your database, keeping your data safe from unauthorized access and preventing malicious users from messing up your database. Rules can be as simple or complex as your app needs. Remember to keep these rules updated as your database structure changes.

Finally, test your schema design to make sure it supports all the queries your app will need to make, returns the expected data, and performs well under expected usage conditions. Handle all potential exceptions and edge cases during testing to ensure your app is robust and reliable.