CONV() function truncates the result to 21 symbol with default charset.

MariaDB [test]> create table t1 (si smallint) engine=columnstore;

Query OK, 0 rows affected (0.480 sec)

MariaDB [test]> drop table t1;

Query OK, 0 rows affected (0.354 sec)

MariaDB [test]> create table t1 (si smallint);

Query OK, 0 rows affected (0.020 sec)

MariaDB [test]> insert into t1 values (-8.76221);

Query OK, 1 row affected (0.004 sec)

MariaDB [test]> select conv(si, 16, 8) from t;

+------------------------+

| conv(si, 16, 8)        |

+------------------------+

| 1777777777777777777767 |

+------------------------+

1 row in set (0.001 sec)

MariaDB [test]> create table t2 as select conv(si, 16, 8) from t;

Query OK, 1 row affected, 1 warning (0.028 sec)

Records: 1  Duplicates: 0  Warnings: 1

MariaDB [test]> show warnings;

+---------+------+------------------------------------------------------+

| Level   | Code | Message                                              |

+---------+------+------------------------------------------------------+

| Warning | 1265 | Data truncated for column 'conv(si, 16, 8)' at row 1 |

+---------+------+------------------------------------------------------+

1 row in set (0.000 sec)

MariaDB [test]> show create table t2

    -> ;

+-------+----------------------------------------------------------------------------------------------------------------------------+

| Table | Create Table                                                                                                               |

+-------+----------------------------------------------------------------------------------------------------------------------------+

| t2    | CREATE TABLE `t2` (

  `conv(si, 16, 8)` varchar(21) CHARACTER SET utf8 DEFAULT NULL

) ENGINE=InnoDB DEFAULT CHARSET=latin1 |

+-------+----------------------------------------------------------------------------------------------------------------------------+

1 row in set (0.000 sec)

MariaDB [test]> select length(conv(si, 16, 8)) from t;

+-------------------------+

| length(conv(si, 16, 8)) |

+-------------------------+

|                      22 |

+-------------------------+

1 row in set (0.001 sec)

MariaDB [test]> create table t1 (si smallint) engine=columnstore;

Query OK, 0 rows affected (0.480 sec)

MariaDB [test]> drop table t1;

Query OK, 0 rows affected (0.354 sec)

MariaDB [test]> create table t1 (si smallint);

Query OK, 0 rows affected (0.020 sec)

MariaDB [test]> insert into t1 values (-8.76221);

Query OK, 1 row affected (0.004 sec)

MariaDB [test]> create table t2 as select conv(si, 16, 8) from t1;

ERROR 1406 (22001): Data too long for column 'conv(si, 16, 8)' at row 1

MariaDB [test]> select length(conv(si, 16, 8)) from t1;

+-------------------------+

| length(conv(si, 16, 8)) |

+-------------------------+

|                      22 |

+-------------------------+

1 row in set (0.001 sec)