MySQL's (innodb) primary key / unique key is restricted to 767 bytes.. currently the url of a web page is used as a primary key in nutch storage.
when using latin1 character set on the 'id' column @ length 767 bytes/characters; unicode characters in urls cause jdbc to throw an exception,
java.io.IOException: java.sql.BatchUpdateException: Incorrect string value: '\xE2\x80\x8' for column 'id' at row 1
when using utf8mb4 character set on the 'id' column @ length 190 characters / 760 bytes to fully support unicode characters; the field length becomes insufficient
It may be better to use a hash of the url as the primary key instead of the url itself. This would allow urls of any length and full utf8 support.