Mastering Row Level Security (RLS) in Databases

Today I spent focused time studying Row Level Security (RLS), a critical capability for protecting data at the database layer instead of relying only on application logic.

What I learned

1. Core concept

• RLS controls access to individual rows based on the authenticated user.
• This prevents unauthorized access to other users’ data (IdOR) at the source.

2. How it works

• Enable RLS with ALTER TABLE ... ENABLE ROW LEVEL SECURITY.
• Define policies in SQL to decide which rows a user can SELECT, INSERT, UPDATE, or DELETE.

3. Multi-tenancy use case

• RLS is a strong foundation for SaaS systems.
• Instead of adding WHERE user_id = ... in every query, the database enforces filtering automatically.

Practical example (PostgreSQL)

-- Enable RLS for the profiles table
ALTER TABLE profiles ENABLE ROW LEVEL SECURITY;

-- Policy: users can only read their own profile
CREATE POLICY user_sel_own_profile ON profiles
    FOR SELECT
    USING (auth.uid() = user_id);

Reflection

Pushing security down into the database makes the system far more robust. Even if application code has a bug, user data remains protected by this final guardrail.

Next goals

• Try RLS with Supabase auth.
• Evaluate performance impact on tables with millions of records.