RDB$INDEX_SEGMENTS stores the segments (table columns) of indexes and
their positions in the key. A separate row is stored for each column in an index.
||The name of the index this segment is related to.
The master record is RDB$INDICES.RDB$INDEX_NAME.
||The name of a column belonging to the index, corresponding
to an identifier for the table and that column in RDB$RELATION_FIELDS.RDB$FIELD_NAME
||The column position in the index. Positions are numbered left-to-right,
starting at zero
||The last known (calculated) selectivity of this column in the index.
The higher the number, the lower the selectivity.