Firebase Retrieving Data in Swift

It gives you that warning message indexOn because you are doing a query.

you should define the keys you will be indexing on via the .indexOn rule in your Security and Firebase Rules. While you are allowed to create these queries ad-hoc on the client, you will see greatly improved performance when using .indexOn

As you know the name you are looking for you can directly go to that node, without a query.

    let ref:FIRDatabaseReference! // your ref ie. root.child("users").child("[email protected]")

    // only need to fetch once so use single event

    ref.observeSingleEventOfType(.Value, withBlock: { snapshot in

        if !snapshot.exists() { return }

        //print(snapshot)

        if let userName = snapshot.value["full_name"] as? String {
            print(userName)
        }
        if let email = snapshot.value["email"] as? String {
            print(email)
        }

        // can also use
        // snapshot.childSnapshotForPath("full_name").value as! String
    })

Swift 4

let ref = Database.database().reference(withPath: "user")
    ref.observeSingleEvent(of: .value, with: { snapshot in

        if !snapshot.exists() { return }

        print(snapshot) // Its print all values including Snap (User)

        print(snapshot.value!)

        let username = snapshot.childSnapshot(forPath: "full_name").value
        print(username!)

    })

{
    "rules": {
         "tbl_name": {
            ".indexOn": ["field_name1", "field_name2"]
         },
    ".read": true,
    ".write": true
    }
}

You can apply indexOn on any field. Add this json in rules security and rules tab. Hope this works for you. :)