The main purpose of a database index in data retrieval is to enhance the speed and efficiency of data queries. An index is a data structure that improves the performance of database searches by allowing the database management system to locate records quickly without scanning the entire dataset. Indexing significantly reduces query execution time, especially in large tables. However, while indexes improve read operations, they can slightly slow down write operations, as the index must be updated with any data modification, necessitating a balance between performance and resource utilization.