10. 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; wherein the step of providing a library of dictionary segments further comprises providing dictionary segments that are adapted for text compression of characters including alphabetic letters, numerals, and spacing and punctuation characters; and wherein the step of scanning records includes keeping statistics indicating the occurrences of characters in the scanned record block and the step of selecting dictionary segments for the system-built dictionary comprises selecting a dictionary segment adapted for compression of one or more characters in accordance with the statistics for those characters in the scanned record block.
10. 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; wherein the step of providing a library of dictionary segments further comprises providing dictionary segments that are adapted for text compression of characters including alphabetic letters, numerals, and spacing and punctuation characters; and wherein the step of scanning records includes keeping statistics indicating the occurrences of characters in the scanned record block and the step of selecting dictionary segments for the system-built dictionary comprises selecting a dictionary segment adapted for compression of one or more characters in accordance with the statistics for those characters in the scanned record block. 14. A method as defined in claim 10, wherein the step of selecting dictionary segments further comprises: identifying a dictionary segment adapted for compression of one or more predetermined characters as a candidate for selection to the system-built dictionary if the dictionary segment is adapted for text compression of one or more characters and the occurrences of the characters in the scanned record block exceeds a predetermined candidate threshold value; evaluating the effectiveness of the candidate dictionary segments during a sampling procedure in which a candidate dictionary segment is applied to a sample of the dataset and sampling statistics are collected on the compression performance of the candidate; and selecting candidate dictionary segments in accordance with the sampling statistics.