Explain PostgreSQL row-level security (RLS).

RLS allows tables to have policies that restrict which rows users can see or modify.

Example:

-- Enable RLS on table
ALTER TABLE documents ENABLE ROW LEVEL SECURITY;

-- Create policy
CREATE POLICY user_documents ON documents
    FOR ALL TO public
    USING (owner_id = current_user_id());

-- Users only see their own documents

Benefits:
• Fine-grained access control
• Transparent to applications
• Multi-tenant applications
• Data privacy compliance