```
CREATE TABLE a
(
    `number` UInt64,
    `x` MATERIALIZED x
)
ENGINE = MergeTree
ORDER BY number;
-- that should fail!

insert into a values (1);
Received exception from server (version 21.7.1):
Code: 47. DB::Exception: Received from localhost:9000. DB::Exception: Missing columns: 'x' while processing query: 'CAST(x, 'UInt8') AS x', required columns: 'x' 'x'. 
```
{
    Names ret;
    for (const auto & col : columns)
        if (col.default_desc.kind != ColumnDefaultKind::Alias)
            ret.emplace_back(col.name);
    return ret;
}
NameAndTypePair ColumnsDescription::getPhysical(const String & column_name) const
{
    auto it = columns.get<1>().find(column_name);
    if (it == columns.get<1>().end() || it->default_desc.kind == ColumnDefaultKind::Alias)
        throw Exception("There is no physical column " + column_name + " in table.", ErrorCodes::NO_SUCH_COLUMN_IN_TABLE);
    return NameAndTypePair(it->name, it->type);
}
NameAndTypePair ColumnsDescription::getPhysicalOrSubcolumn(const String & column_name) const
{
    if (auto it = columns.get<1>().find(column_name); it != columns.get<1>().end()
        && it->default_desc.kind != ColumnDefaultKind::Alias)
    {
        return NameAndTypePair(it->name, it->type);
    }
    if (auto it = subcolumns.find(column_name); it != subcolumns.end())
    {
        return it->second;
    }
    throw Exception(ErrorCodes::NO_SUCH_COLUMN_IN_TABLE,
        "There is no physical column or subcolumn {} in table.", column_name);
}
bool ColumnsDescription::hasPhysical(const String & column_name) const