create table t1 (a bigint, b bigint)engine=columnstore;
create table t2 (a bigint, b bigint)engine=columnstore;
insert into t1 values (5,5),(4,4),(3,3);
insert into t1 values (5,5),(4,4),(3,3);

SELECT * FROM(
SELECT * FROM (
select * from (SELECT a, b FROM t1 ORDER BY b LIMIT 100) sq1
UNION ALL
select * from (SELECT a, b FROM t2 ORDER BY b LIMIT 100) sq2) sq3
order by sq3.a limit 999) sq4;

This query produces expected results. LimitedOrderBy got called three times as expected. And if I add ORDER BY at the top level then filesort comes into play.
I will get back to the original data set and problem description.

Please review the change.

For QA: Here are the steps to reproduce the issue.

create table t1 (a bigint, b bigint)engine=columnstore;
create table t2 (a bigint, b bigint)engine=columnstore;
insert into t1 values (5,5),(4,4),(3,3);
insert into t1 values (5,5),(4,4),(3,3);

SELECT * FROM
((SELECT a, b FROM t1 ORDER BY b LIMIT 2) sq1
UNION ALL
(SELECT a, b FROM t2 ORDER BY b LIMIT 2) sq2)
ORDER BY 1,2;

Reproduced the issue on 1.2.3-1

It is not yet fixed in last nights 1.2.4-1 nightly. It looks like it was merged after the build was made.

The SELECT statement in the last comment has an extra comma in the 2nd subquery.

Build verified: 1.2.4-1 nightly

server commit:
137b9a8
engine commit:
b8de456

Reproduced incorrect result issue in 1.2.3-1:

Welcome to the MariaDB monitor. Commands end with ; or \g.
Your MariaDB connection id is 13
Server version: 10.3.13-MariaDB-log Columnstore 1.2.3-1

Copyright (c) 2000, 2018, Oracle, MariaDB Corporation Ab and others.

Type 'help;' or '\h' for help. Type '\c' to clear the current input statement.

MariaDB [mytest]> SELECT * FROM ((SELECT a, b FROM t1 ORDER BY b LIMIT 2) UNION ALL (SELECT a, b FROM t2 ORDER BY b LIMIT 2)) as sq3 ORDER BY 1,2;
----------+

----------+

----------+
2 rows in set (0.110 sec)

1.2.4-1

Welcome to the MariaDB monitor. Commands end with ; or \g.
Your MariaDB connection id is 14
Server version: 10.3.13-MariaDB-log Columnstore 1.2.4-1

Copyright (c) 2000, 2018, Oracle, MariaDB Corporation Ab and others.

Type 'help;' or '\h' for help. Type '\c' to clear the current input statement.

MariaDB [mytest]> SELECT * FROM ((SELECT a, b FROM t1 ORDER BY b LIMIT 2) UNION ALL (SELECT a, b FROM t2 ORDER BY b LIMIT 2)) as sq3 ORDER BY 1,2;
----------+

----------+

----------+
2 rows in set (0.033 sec)