1. A computer-implemented method of forming system-built dictionaries used in the compression of uninterrupted character byte strings in the management of catalog-based data sets comprising records located in external storage, said catalogs containing information identifying the format of each data set and its location in the external storage, each data set assuming a closed status until subject to access commands, and thereupon assuming an open status, the method comprising the steps of: (a) providing a library of dictionary segments; (b) scanning the records during the initial recordation of records to the external storage and collecting characteristic attributes and statistics from the scanning; (c) selecting dictionary segments from the library of dictionary segments in accordance with the attributes and statistics; and (d) combining the dictionary segments to form a system-built dictionary.