You are tasked with designing a BigQuery data warehouse aimed at analyzing sales data for a telecommunication service provider. The primary requirement is to create an effective data model encompassing customers, products, and subscriptions. Each of these entities can be updated monthly, and it is crucial to maintain a comprehensive historical record of all changes. The designed data model would serve both current and historical reporting needs through a visualization layer. Your goal is to ensure that the data model remains simple, user-friendly, and cost-efficient. What approach should you take?