Pass virtual column from inner table to materialized view

[filimonov]

create table x engine=MergeTree ORDER BY tuple() as select * from numbers(100);
create materialized view x_mv engine=MergeTree ORDER BY tuple() as select * from x;
insert into x select * from numbers(100);

That works:

select _part, count() from `.inner.x_mv` group by _part;

SELECT 
    _part, 
    count()
FROM `.inner.x_mv`
GROUP BY _part

┌─_part─────┬─count()─┐
│ all_1_1_0 │     100 │
└───────────┴─────────┘

1 rows in set. Elapsed: 0.004 sec.

That - not:

select _part, count() from x_mv group by _part;
Received exception from server (version 20.5.1):
Code: 47. DB::Exception: Received from localhost:9000. DB::Exception: Missing columns: '_part' while processing query: 'SELECT _part, count() FROM x_mv GROUP BY _part', required columns: '_part', source columns: 'number'. 

[lingtaolf]

@alexey-milovidov I'm interested in this and I hope to work on this.