A compressed record image contains only the key (primary, unique, KEYCOLS) and the columns that changed in the processed row. By default, trail records written by processes on Windows and UNIX systems are always compressed. The format of a compressed record is as follows:

column_index column_length column_data[...]

Where:

• column_index is the ordinal index of the column within the source table (2 bytes).

• colum_length is the length of the data (2 bytes).

• column_data is the data, including NULL or VARCHAR length indicators.

Enscribe records written from the NonStop platform may be compressed. The format of a compressed Enscribe record is as follows:

field_offset field_length field_value[...]

Where:

• field_offset is the offset within the original record of the changed value (2 bytes).

• field_length is the length of the data (2 bytes).

• field_value is the data, including NULL or VARCHAR length indicators.

The first field in a compressed Enscribe record is the primary or system key.