The field index, or .fdx file.
This contains, for each document, a pointer to its field data, as follows:
FieldIndex (.fdx) --> <FieldValuesPosition> SegSize
FieldValuesPosition --> Uint64
This is used to find the location within the field data file of the
fields of a particular document. Because it contains fixed-length
data, this file may be easily randomly accessed. The position of
document n 's field data is the Uint64 at n*8 in this file.