MV union all workaround

several_MV_one_internal_store

create table tableA (A String) Engine=MergeTree order by tuple();
create table tableB (B String) Engine=MergeTree order by tuple();
create table tableC (C String) Engine=Null;
create table storeABC(ABC String) Engine=MergeTree order by tuple();
create materialized view MVA to storeABC as select A ABC from tableA;
create materialized view MVB to storeABC as select B ABC from tableB;
create materialized view MVC to storeABC as select C ABC from tableC;
insert into tableA values('A');
insert into tableB values('B');
insert into tableC values('C');

select * from storeABC;
┌─ABC─┐
│ A   │
│ B   │
│ C   │
└─────┘

select * from MVC; (the same store storeABC -> the same result)
┌─ABC─┐
│ A   │
│ B   │
│ C   │
└─────┘