1. A computer-implemented method of self-tuning a database management system in response to collecting performance statistics measuring an operational performance of the database management system, the method comprising: selecting, with a workload repository module that collects performance statistics from a shared memory of a database kernel of a database management system provided by a set of one or more computer systems and that executes as an internal system process of the database management system, one or more categorical types of performance statistics to capture within the database management system from a plurality of categorical types of performance statistics, wherein each of the performance statistics is organized into a categorical type of the plurality of categorical types and a level of the categorical type, the level being a subset of the categorical type; capturing a set of performance statistics from the shared memory of the database kernel of the database management system with the workload repository module according to the one or more selected categorical types of performance statistics, wherein each captured performance statistic of the set of performance statistics is organized into a categorical type that matches the one or more selected categorical types of performance statistics; storing the set of performance statistics by the workload repository module in a database of the database management system based on a database schema configured for performance statistics; extracting a subset of the set of performance statistics from the database of the database management system by a database client executing as an internal system process of the database management system provided by the one or more computer systems according to the database schema for performance statistics; analyzing the subset of the set of performance statistics with the database client according to one or more tuning models provided by the database client; and tuning the operational performance of the database management system in response to a result of the analysis of the subset of the set of performance statistics by the database client.