Multi-tenant data isolation in SaaS is routinely done wrong and causes costly rewrites when first enterprise customers arrive

Most SaaS products start single-tenant and bolt on multi-tenancy later, but the database model chosen on day one becomes the hardest decision to change. PostgreSQL row-level security has side-channel leakage issues (recent CVEs); shared-schema tenancy breaks when one tenant's heavy queries degrade others; per-tenant databases cause thousands of migration headaches at scale. A YC-backed company (Fortress, S24) raised specifically to solve this, launching on HN to 121 points and 24 comments in September 2024. Multiple 2026 engineering guides document teams choosing wrong and spending 3+ months on complete data migrations. The pain sharpens when an enterprise deal arrives with a SOC 2 demand for tenant isolation that the current schema cannot satisfy without a full rewrite.