Prolog 1 files and Prolog 2 files operate identically. If an indexed file uses only string data-type keys, the file is a Prolog 1 file. If an indexed file uses numeric type keys, it is a Prolog 2 file. The compression allowed with Prolog 3 files is not possible with Prolog 1 or Prolog 2 files. They also give you the option of saving space by compressing your data, indexes, and keys.
Key compression compresses the key values in the data buckets.
Likewise, index compression compresses the key values in index buckets, and data compression compresses the data portion of the records in the data buckets. Key or index compression is restricted to the string key data type and the string must be at least 6 bytes in length. You cannot use key compression or index compression with any numeric or collated key data types. With key or index compression, repeating leading and trailing characters are compressed. With front key compression, any characters that are identical to the characters at the front of the previous key are compressed.