With front key compression, any characters that are identical to the characters at the front of the previous key are compressed. With rear key compression, any repeating characters at the end of the key are compressed to a single character. Enabling index compression results in RMS doing a sequential search in index buckets rather than its default binary search, since each index key value must be expanded until a match is found. With data compression, RMS can compress sequences of up to 255 repeating characters in the data portion of the user data records. For optimal performance, RMS does not compress sequences having less than five repeating characters.
Compression has a direct effect on CPU time and disk space.
Compression increases CPU time, but the keys are smaller, so your application can scan more quickly through the data and index buckets. The disk space saved by using Prolog 3 indexed files can significantly improve performance. Prolog 3 files can have segmented primary keys, but the segments cannot overlap. If you want to use a Prolog 3 file in this case, consider defining the overlapping segmented key as an alternate key and choosing a different key to be the primary key. If you want to use overlapping primary key segments, you must use a Prolog 2 file.