The RDB$RELATION_CONSTRAINT_FLDS system table lists the columns
    that participate in unique, primary, or foreign key declarations
    for table-specific constraints.

    There is one row for each column that represents all or part of a
    unique, primary, or foreign key constraint.

    The following table provides information on the columns of the
    RDB$RELATION_CONSTRAINT_FLDS system table.

                                Data
    Column Name                 Type      Summary Description

    RDB$CONSTRAINT_NAME         char(31)  The name of a constraint
                                          for which the specified
                                          column participates.
    RDB$FIELD_NAME              char(31)  The name of the column
                                          that is all or part of the
                                          specified constraint. The
                                          value in this column is the
                                          same as that stored in the
                                          RDB$RELATION_FIELDS system
                                          table.
    RDB$FIELD_POSITION          integer   The ordinal position of the
                                          specified column within the
                                          column list that declares
                                          the unique, primary or
                                          foreign key constraint.

                                          For column-level
                                          constraints, there will
                                          always be only one column
                                          in the list. The first
                                          column in the list has
                                          position value 1, the
                                          second has position value
                                          2, and so on.
    RDB$FLAGS                   integer   Reserved for future use.
    RDB$SECURITY_CLASS          char(20)  Reserved for future use.