Columns longer than 256 bytes will be created as 256 byte Alphanumeric fields.
SELECT and FETCH will only be able to access the first 256 bytes (if there are more than 256 bytes, a warning message will be usually issued). This means that UPDATEs of rows containing such a column will be TRUNCATED to 256. This may result in data being lost that is updated by an external application.
It is highly recommended that you use these tables for reading only. Alternatively, you may delete the column from the table.