Abstract: A method of maintaining a database index. The index comprises a hierarchical structure of conclusion sets arranged in a series of levels. The method comprising: inserting an “Insert” conclusion set entry into a high level conclusion set; migrating the “Insert” conclusion set entry from the high level conclusion set to a low level conclusion set; deleting the “Insert” conclusion set entry by inserting a “Delete” conclusion set entry into the high level conclusion set; and migrating the “Delete” conclusion set entry from the high level conclusion set to the low level conclusion set whilst maintaining the conclusion set entries in chronological order of insertion within each conclusion set and between levels of conclusion sets.