21.7.3 throws an exception if `_sample_factor` is used in a query:
```
CREATE TABLE sessions
(
  `user_id` UInt64
)
ENGINE = MergeTree
ORDER BY user_id 
SAMPLE BY user_id;

insert into sessions values(1);

SELECT
    sum(user_id * _sample_factor) 
FROM sessions
SAMPLE 10000000

DB::Exception: Cannot find column `user_id` in source stream (version 21.8.1.7409 (official build))

SELECT
    sum(user_id) 
FROM sessions
SAMPLE 10000000

sum(user_id)
------------
           1
```
No exception on 21.6.5
{
    switch (type)
    {
        case ReadFromMergeTree::IndexType::None:
            return "None";
        case ReadFromMergeTree::IndexType::MinMax:
            return "MinMax";
        case ReadFromMergeTree::IndexType::Partition:
            return "Partition";
        case ReadFromMergeTree::IndexType::PrimaryKey:
            return "PrimaryKey";
        case ReadFromMergeTree::IndexType::Skip:
            return "Skip";
    }
    __builtin_unreachable();
}
static const char * readTypeToString(ReadFromMergeTree::ReadType type)
{
    switch (type)
    {
        case ReadFromMergeTree::ReadType::Default:
            return "Default";
        case ReadFromMergeTree::ReadType::InOrder:
            return "InOrder";
        case ReadFromMergeTree::ReadType::InReverseOrder:
            return "InReverseOrder";
    }
    __builtin_unreachable();
}
void ReadFromMergeTree::describeActions(FormatSettings & format_settings) const