The main function of a database index is to improve the speed of data retrieval operations on a database. By creating a data structure that allows faster lookup, indexes help optimize query performance efficiently. Without indexes, the database system would have to scan every record to locate the desired data, which can be time-consuming, especially for large datasets. Indexes can be applied to one or multiple columns and can significantly reduce the time taken for search operations. However, maintaining indexes also incurs overhead during data modifications, so a balance must be struck between performance and maintenance.