Row Level Security (RLS) is a database feature that automatically filters table rows based on who's accessing them.

Instead of creating separate tables or views for different users, you write a policy once that says "sales reps only see their own region's data" or "managers see their team's records." The database enforces this invisibly on every query.

Most builders use RLS in PostgreSQL, Supabase, or Power BI to build multi-tenant SaaS apps where customers share infrastructure but can't see each other's data. It's cleaner than filtering in application code because you can't accidentally forget to add the WHERE clause.

Free in PostgreSQL and Supabase. SQL Server and Oracle have it built-in. Some BI tools like Power BI include it for report-level security.