Abstract: The invention concerns the compression, querying and updating of tree structured data. For example, but not limited to, the invention concerns a synopsis (16) of a database system that is used in the selection of the optimal execution plan (10) for a query (8). Compression is based on representing the data as a set of definition and compressing the data by consolidating the number of definitions. A selectivity estimate can be determined based on this compressed representation, including a maximum and minimum selectivity count. The invention also provides a way to update the compressed version of the tree data without uncompressing large amounts of the compressed data unnecessarily. Aspects of the invention are methods, computer systems and software for performing the invention.