The problem was already described in

MySQL defines "effective" maximum lengths of text columns. "Effective" in the case means the length in bytes, not in characters. If the database charset is utf-8 (which nowadays is default in almost all cases), mysql allows the creation of columns of 1/3 of the defined length.

The correct lengths for varchar fields are:

0...21845 -> Varchar(length)
21846...5592405 -> mediumtext
more -> longtext

So the correct implementation of registerVarcharTypes is:

	protected void registerVarcharTypes() {
		registerColumnType( Types.VARCHAR, "longtext" );
		registerColumnType( Types.VARCHAR, 16777215/3, "mediumtext" ); // if UTF-8 tables are used, only 1/3 of the max length is available
		registerColumnType( Types.VARCHAR, 65535/3, "varchar($l)" ); // if UTF-8 tables are used, only 1/3 of the max length is available
		registerColumnType( Types.LONGVARCHAR, "longtext" );
	}

This fix is independent of any rewrite of the length-logic! It is nessecary to make hibernate's mysql(5) dialect work correctly.

The workaround in