ctas with outer join like
```sql
create table a (
id int not null,
name varchar(20) not null
)
distributed by hash(id) buckets 4
properties (
"replication_num"="1"
);
create table b (
id int not null,
age int not null
)
distributed by hash(id) buckets 4
properties (
"replication_num"="1"
);
insert into a values(1, 'ww'), (2, 'zs');
insert into b values(1, 22);
create table c properties("replication_num"="1") as select a.id, a.name, b.age from a left join b on a.id = b.id;
```
the column 'age' in c is not null, but nullable is expected, we fix it by use the nullable mode of the outputs of root plan fragment.
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