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 end() const { return impl.rows; }
    bool atEnd() const { return impl.getRow() >= impl.rows; }
    void nextN(size_t num) { impl.getPosRef() += num; }
    void setCompareNullability(const MergeJoinCursor & rhs)
    {
        has_left_nullable = false;
        has_right_nullable = false;
        for (size_t i = 0; i < impl.sort_columns_size; ++i)
        {
            has_left_nullable = has_left_nullable || isColumnNullable(*impl.sort_columns[i]);
            has_right_nullable = has_right_nullable || isColumnNullable(*rhs.impl.sort_columns[i]);
        }
    }
    Range getNextEqualRange(MergeJoinCursor & rhs)
    {
        if (has_left_nullable && has_right_nullable)
            return getNextEqualRangeImpl<true, true>(rhs);
        else if (has_left_nullable)
            return getNextEqualRangeImpl<true, false>(rhs);
        else if (has_right_nullable)
            return getNextEqualRangeImpl<false, true>(rhs);
        return getNextEqualRangeImpl<false, false>(rhs);
    }
    int intersect(const Block & min_max, const Names & key_names)