Postgres Row-Level Security for Multi-Tenant Applications

Every multi-tenant SaaS I've worked on eventually hits the same incident: a developer forgets a WHERE tenant_id = $1 clause in a new query, and suddenly tenant A can see tenant B's invoices. Code review catches most of these, but not all — and "most" isn't good enough when the data is customer PII or financial records.

The fix isn't more discipline. It's moving tenant isolation out of application code and into the database itself. Postgres Row-Level Security (RLS) lets you define policies that filter rows automatically, so even a query with a missing WHERE clause can't leak data across tenants. It's not a silver bullet, but it's the closest thing to one for this specific problem.