The developers of Checkly, a SaaS company, chose to use PostgreSQL as their database management system due to its excellent engineering and features that meet the needs of their application. They considered using MongoDB or DynamoDB, but ultimately found them unsuitable for their relational data model. The team opted for a single, multi-tenant database with separate schemas for each customer, which provides scalability, security, and ease of migration. Their user and account management system is designed to handle multiple users per account, account settings, and access rights, utilizing an enum field for role-based access control and a string array type for feature toggles. The plans and subscriptions table has a weak relation with the accounts table, but both use the same fields for plan limits and features, allowing for easy customization and experimentation with pricing and plan composition.