What is RLS

RLS (Row-Level Security) is a database feature that applies security policies at the row level, so users only see the data they`re allowed to see. That access is often decided by roles, credentials, or other contextual factors like department or manager relationships.

How It Works

RLS sets rules at the database layer. When a user runs a query, the database checks the policies for each row and either allows or blocks it. This differs from application-layer filters, which rely on developers to enforce the rules. Built-in RLS is more consistent and less prone to mistakes.

Technical Details

PostgreSQL and SQL Server both provide RLS features. You typically enable RLS on a table, then create one or more row-level policies that define which rows a user or role can read, update, or insert. The database then applies these policies automatically at query time, preventing rows from returning if they fail the policy criteria.

How to Write It

Basic Syntax

-- Example of Row-Level Security in action:

-- 1. Create a table
CREATE TABLE customer_data (
  id SERIAL PRIMARY KEY,
  customer_id INT,
  data TEXT,
  created_by VARCHAR(100)
);

-- 2. Turn on row-level security
ALTER TABLE customer_data ENABLE ROW LEVEL SECURITY;

-- 3. Policy: Users can see only their own rows
CREATE POLICY user_isolation_policy ON customer_data
  FOR ALL
  USING (created_by = current_user);

-- 4. Policy: Managers can see their team's data
CREATE POLICY manager_policy ON customer_data
  FOR ALL
  USING (
    EXISTS (
      SELECT 1 FROM employee_hierarchy
      WHERE manager_username = current_user
      AND employee_username = created_by
    )
  );

-- 5. Policy: A specific role can see assigned customers
CREATE POLICY customer_service_policy ON customer_data
  FOR SELECT
  TO customer_service_role
  USING (
    EXISTS (
      SELECT 1 FROM customer_assignments
      WHERE agent_username = current_user
      AND customer_id = customer_data.customer_id
    )
  );

Supported Platforms

PostgreSQL

Available from version 9.5 onwards, with flexible policy definitions and a straightforward syntax.

SQL Server

Introduced native Row-Level Security in SQL Server 2016, configurable via security predicates.

Learn More

Best Practices

Keep policies simple and modular. Split them by use case if needed.
Document your RLS rules so other developers understand how data is being restricted.
Regularly test your RLS policies with multiple user roles to confirm the correct data is shown or hidden.
Pair RLS with encryption or role-based permissions for stronger security.

Common Pitfalls

Forgetting to enable RLS on the table, which leaves policies unused.
Relying on the application layer alone without a database-enforced fallback.
Complex policy logic that confuses future developers or leads to unexpected data denial.

Advanced Tips

Use separate policies for SELECT, INSERT, UPDATE, and DELETE, to give more precise control.
Combine RLS with table partitioning for large-scale systems that have strict data isolation needs.
Use auditing or logging to track policy hits and misses for security reviews.