A special Oracle Rdb data type designed to handle large pieces
    of data with a segmented internal structure. The maximum size
    of a string segment is 64K bytes. Except for the length of the
    string's segments, Oracle Rdb does not know anything about the type
    of data contained in a segmented string. In a segmented string,
    you can store large amounts of text, long strings of binary input
    from a data collecting device, or graphic data. A program can
    then extract the data from the database and handle it in the
    appropriate way.

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    Because Oracle Rdb does not know what kind of data is contained in
    a segmented string, you cannot perform many of the standard data
    manipulation functions on it. You cannot use logical operators,
    such as EQ and CONTAINING, to compare segmented strings. Oracle Rdb
    does not perform any data type conversion on data that is
    transferred into or out of a segmented string. You can use the
    MISSING operator on a segmented string field.

    Oracle Rdb defines a special name to refer to the segments of a
    segmented string. This name is equivalent to a field name;
    it names the "fields" or segments of the string. Furthermore,
    because segments can vary in length, Oracle Rdb also defines a name
    for the length of a segment. You must use these names in the
    value expressions that you use to retrieve the length and value
    of a segment. These names are:

    RDB$VALUE          The value stored in a segment of a segmented
                       string
    RDB$LENGTH         The length in bytes of a segment

    Because a single segmented string field value is made up of
    multiple segments, you must manipulate the segments one at a
    time. Therefore, segmented string operations require an internal
    looping mechanism, much like the record stream set up by a FOR or
    START_STREAM statement.