Strange. I ran the query multiple times. One time it worked.

Build verified: 1.2.4-1 nightly

[dlee@master centos7]$ cat gitversionInfo.txt
server commit:
137b9a8
engine commit:
b3a7559

Default installation does have avg_mode() install. It needs to be installed manually.

MariaDB [mytest]> CREATE AGGREGATE FUNCTION avg_mode returns REAL soname 'libudf_mysql.so';
Query OK, 0 rows affected (0.002 sec)

Reproduced the issue in 1.2.2-1

MariaDB [mytest]> select c1,count(distinct c2) as c2,count(distinct c3) as c3, avg_mode(c4) as c4 from t1 group by c1;
ERROR 1305 (42000): FUNCTION mytest.avg_mode does not exist
MariaDB [mytest]> CREATE AGGREGATE FUNCTION avg_mode returns REAL soname 'libudf_mysql.so';
Query OK, 0 rows affected (0.002 sec)

MariaDB [mytest]> select c1,count(distinct c2) as c2,count(distinct c3) as c3, avg_mode(c4) as c4 from t1 group by c1;
ERROR 1815 (HY000): Internal error: An unexpected condition within the query caused an internal processing error within Columnstore. Please check the log files for more details. Additional Information: error in TupleAggregateSte

Verified in 1.2.4-1 nightly

MariaDB [mytest]> create table t1 (c1 int, c2 int, c3 int, c4 int)engine=columnstore;
Query OK, 0 rows affected (0.115 sec)

MariaDB [mytest]> insert into t1 values (1,2,3,4),(1,2,3,4),(1,2,3,4),(1,2,3,4);
Query OK, 4 rows affected (0.278 sec)
Records: 4 Duplicates: 0 Warnings: 0

MariaDB [mytest]> select c1,count(distinct c2) as c2,count(distinct c3) as c3, avg_mode(c4) as c4 from t1 group by c1;
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1 row in set (0.078 sec)