What is a writeable CTE in PostgreSQL?

A writeable CTE contains DML (INSERT, UPDATE, DELETE) in the CTE body and uses RETURNING to pass modified data to the main query or other CTEs. Example: WITH updated AS (UPDATE employees SET salary = salary * 1.1 WHERE dept_id = 1 RETURNING emp_id, salary) INSERT INTO audit(emp_id, new_salary) SELECT emp_id, salary FROM updated. Both UPDATE and INSERT execute in a single transaction — atomic. The