The RDB$INDEX_SEGMENTS system table describes the columns that
    make up an index's key. Each index must have at least one column
    within the key. The following table provides information on the
    columns of the RDB$INDEX_SEGMENTS system table.

                                Data
    Column Name                 Type      Summary Description

    RDB$INDEX_NAME              char(31)  The name of the index
                                          of which this row is a
                                          segment.
    RDB$FIELD_NAME              char(31)  The name of a column that
                                          participates in the index
                                          key. This column name
                                          matches the name in the
                                          RDB$FIELD_NAME column of
                                          the RDB$RELATION_FIELDS
                                          table.
    RDB$FIELD_POSITION          integer   The ordinal position of
                                          this key segment within
                                          the total index key. No
                                          two segments in the key may
                                          have the same RDB$FIELD_
                                          POSITION.
    RDB$FLAGS                   integer   A bit mask where Bit 0
                                          is set for descending
                                          segments, otherwise the
                                          segments are ascending.
    RDB$FIELD_LENGTH            integer   Shortened length of text
                                          for compressed indexes.
    RDBVMS$FIELD_MAPPING_LOW    integer   Shows the lower limit of
                                          the mapping range.
    RDBVMS$FIELD_MAPPING_HIGH   integer   Shows the higher limit of
                                          the mapping range.
    RDB$SECURITY_CLASS          char(20)  Reserved for future use.
    RDB$CARDINALITY             bigint    Prefix cardinality for this
                                          and all prior key segments
                                          (assumes sorting by ordinal
                                          position).