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.
{
    if (is_in_memory)
    {
        std::unique_lock lock(rwlock);
        if (!is_in_memory)
        {
            disk_writer->insert(std::move(block));
            return true;
        }
        right_blocks.insert(std::move(block));
        bool has_memory = size_limits.softCheck(right_blocks.row_count, right_blocks.bytes);
        if (!has_memory)
        {
            initRightTableWriter();
            is_in_memory = false;
        }
    }
    else
        disk_writer->insert(std::move(block));
    return true;
}
Block MergeJoin::modifyRightBlock(const Block & src_block) const
{
    Block block = materializeBlock(src_block);
    JoinCommon::removeLowCardinalityInplace(block, table_join->keyNamesRight());
    return block;
}
bool MergeJoin::addJoinedBlock(const Block & src_block, bool)