while the registerVarcharTypes in the MySQLDialect only creates varchar columns with a length up to 255
seems like a regression caused by this commit:
MySQL throws the following error:
Error Code: 1118
Row size too large. The maximum row size for the used table type, not counting BLOBs, is 65535. You have to change some columns to TEXT or BLOBs
when trying to create a table with 3 varchar (20000) colums
Default column sizes will be reworked in HHH-6045. Until then, please explicitly define column lengths on your varchars or use a custom dialect that extends/overrides MySQL5Dialect.