1. A method comprising: distinguishing between different types of long-running queries executing in a database system comprising a computer using a taxonomy of query performance problems, the taxonomy comprising a threshold, an estimated execution time calculated by a query optimizer for a query, an actual execution time of a query, and an elapsed time, wherein distinguishing between the different types of long-running queries using the taxonomy comprises: identifying a respective query as a long-running query in response to an elapsed time for the respective query exceeding the threshold, identifying a first long-running query of the long-running queries as a first type long-running query responsive to determining that an actual execution time of the first long-running query is less than an estimated execution time calculated by the query optimizer for the first long-running query and is less than an elapsed time of the first long-running query but greater than a specified time that is below the elapsed time of the first long-running query, and identifying a second long-running query of the long-running queries as a second, different type long-running query responsive to determining that an actual execution time of the second long-running query is greater than an estimated execution time calculated by the query optimizer for the second long-running query and is less than a specified time that is below an elapsed time of the second long-running query; implementing, by the database system, a remedial action for at least one of the long-running queries.