"so hard"

gistfile1.java

class MyDialectResolver implements DialectResolver {
    public Dialect resolveDialect(DatabaseMetaData metaData) {
        if ( "H2".equals( metaData.getDatabaseProductName ) ) {
            return MyH2Dialect.INSTANCE;
        }
        else if ( "PostgreSQL".equals( metaData.getDatabaseProductName ) ) {
            return MyPgsqlDialect.INSTANCE;
        }
        return null;
    }
}

class MyH2Dialect extends H2Dialect {
    public static final MyH2Dialect INSTANCE = new MyH2Dialect();

    private MyH2Dialect() {
        super();
        registerFunction( "datediff", ... );
    }
}

...