Thankfully "partial merge join" is disabled by default.

```
milovidov-desktop :) SET max_block_size = 6, join_algorithm = 'partial_merge'

SET max_block_size = 6, join_algorithm = 'partial_merge'

Query id: 600704b7-b455-4726-8a9c-8e45b509f2fb

Ok.

0 rows in set. Elapsed: 0.002 sec. 

milovidov-desktop :) SELECT blockSize() bs FROM (SELECT 1 s) js1 ALL RIGHT JOIN (SELECT arrayJoin([2, 2, 2, 2, 2, 2, 2, 3, 3, 3, 3, 3, 3, 3]) s) js2 USING (s) GROUP BY bs ORDER BY bs;

SELECT blockSize() AS bs
FROM
(
    SELECT 1 AS s
) AS js1
ALL RIGHT JOIN
(
    SELECT arrayJoin([2, 2, 2, 2, 2, 2, 2, 3, 3, 3, 3, 3, 3, 3]) AS s
) AS js2 USING (s)
GROUP BY bs
ORDER BY bs ASC

Query id: ae51c589-4b69-4e90-a3a9-7ef4ead72af9

→ Progress: 2.00 rows, 2.00 B (18.21 rows/s., 18.21 B/s.)
```

Found by stress test.
{
        size_t pos = impl.getRow() + 1;
        for (; pos < impl.rows; ++pos)
            if (!samePrev(pos))
                break;
        return pos - impl.getRow();
    }
    
    bool ALWAYS_INLINE samePrev(size_t lhs_pos) const
    {
        for (size_t i = 0; i < impl.sort_columns_size; ++i)
            if (impl.sort_columns[i]->compareAt(lhs_pos - 1, lhs_pos, *(impl.sort_columns[i]), 1) != 0)
                return false;
        return true;
    }
};
namespace
{
MutableColumns makeMutableColumns(const Block & block, size_t rows_to_reserve = 0)