Add support for ALTER TABLE ... CONVERT TO CHARACTER SET

CREATE TABLE tab (

`id` int(11) DEFAULT NULL,

`str` varchar(50) DEFAULT NULL

) ENGINE=Columnstore DEFAULT CHARSET=latin1;

ALTER TABLE tab CONVERT TO CHARACTER SET 'utf8' COLLATE 'utf8_general_ci';

MariaDB [db1]> CREATE TABLE tab (

    -> `id` int(11) DEFAULT NULL,

    -> `str` varchar(50) DEFAULT NULL

    -> ) ENGINE=Columnstore DEFAULT CHARSET=latin1;

Query OK, 0 rows affected (0.19 sec)

MariaDB [db1]> ALTER TABLE tab CONVERT TO CHARACTER SET 'utf8' COLLATE 'utf8_general_ci';

ERROR 1178 (42000): The storage engine for the table doesn't support The syntax or the data type(s) is not supported by Columnstore. Please check the Columnstore syntax guide for supported syntax or data types.