Horizontal scaling entails adding more servers to handle the increased load. By distributing the load across multiple servers, the system can better handle higher volumes of traffic or data processing. This approach contrasts with vertical scaling, where additional resources are added to a single server to enhance its capacity.

Regularly reviewing and updating access controls ensures that only authorized users have appropriate access to the database. Access controls include permissions, privileges, and restrictions set by administrators to manage who can view, modify, or interact with data within the database system.

Scalability testing ensures that a database system can handle a growing amount of workload without compromising performance or stability. Many database testing tools include built-in support for scalability testing.

Ensuring user interface responsiveness is not typically a challenge in database testing. Database testing primarily deals with verifying data integrity, consistency, and accuracy, rather than assessing user interface responsiveness, which falls under other types of testing such as GUI testing or usability testing.

Knowledge-Based Authentication verifies a user's identity based on something they know, such as a password, PIN, or passphrase. It is one of the most commonly used authentication methods and is widely implemented in various systems and applications. However, it is important for users to choose strong passwords and for systems to implement secure password storage mechanisms to prevent unauthorized access.

A self-join in SQL is used when you want to combine rows from the same table based on a related column. It is commonly used to retrieve hierarchical data or to compare rows within the same table. For example, when you have an employee table where each row contains an employee and their manager, you can use a self-join to retrieve the manager's details for each employee.

Database query optimization in performance testing aims to enhance the efficiency of database operations by refining SQL queries, indexing strategies, and database schema to reduce execution time and resource consumption. Optimized queries lead to faster data retrieval and processing, improving overall system performance.

DbUnit is a commonly used tool for automating database tests in a CI environment. It provides a set of functionalities for setup, execution, and verification of database tests, allowing developers to write automated tests for database interactions.

Database testing helps in detecting and rectifying data redundancy, which refers to the unnecessary repetition of data in the database. Removing redundant data improves efficiency, reduces storage space, and enhances data quality.

In a CI/CD environment, database tests are executed automatically whenever there is a code change. This ensures that any modifications to the database schema or queries are immediately validated, maintaining the integrity and reliability of the database within the continuous integration pipeline.