Developers often refactor code to improve the maintainability and comprehension of the software. There are many studies on refactoring activities in traditional software systems. However, refactoring in data-intensive systems is not well explored. Understanding the refactoring practices of developers is important to develop efficient tool support. We conducted a longitudinal study of refactoring activities in data-access classes using 29 SQL and NoSQL database based data-intensive systems. We investigated the prevalence, co-occurrence, and evolution of data-access refactorings, and the association of data-access refactorings with data-access smells. We also conducted a manual analysis of 500 samples of data-access refactoring instances to identify the functionalities of the code that are targeted by such refactorings. Furthermore, we analyzed 500 sample data-access refactoring commits to understand the context behind the applied refactorings and explored the characteristics and contribution of developers involved in the refactorings. We also conducted a developer survey to complement our analysis on the subject systems. Our results show that data-access refactorings are prevalent and different in type. Most of the data-access refactorings target codes that implement data fetching and insertion, but they mostly do not modify data-access queries. Most of the data-access refactorings are done when adding or modifying features and during bug fixes. data-access refactoring is often performed by developers with higher development and refactoring experience. Overall, the results show that data-access refactorings focus on improving the code quality but not optimizing the underlying data-access operations by fixing data-access smells. Hence, more work is needed from the research community on providing awareness and support to practitioners on the benefits of addressing data-access smells with refactorings.