**How to reproduce**
Clickhouse server 20.13.1.5273
```
CREATE TABLE default.test
(
`key` UInt32,
`ts` DateTime CODEC(DoubleDelta, LZ4),
`db_time` DateTime DEFAULT now() COMMENT 'spout-ignore' CODEC(DoubleDelta, LZ4)
)
ENGINE = MergeTree
PARTITION BY toStartOfTenMinutes(db_time)
ORDER BY (key, ts)
TTL db_time + toIntervalHour(3)
SETTINGS index_granularity = 8192, merge_with_ttl_timeout = 3600, min_rows_for_compact_part = 1000000, min_bytes_for_compact_part = 200000000, in_memory_parts_enable_wal = 0;


INSERT INTO test(key, ts) SELECT number % 1000, now() + intDiv(number,1000) FROM numbers(500);

SELECT * FROM test;
500 rows in set. Elapsed: 0.003 sec.

DETACH TABLE test;

ATTACH TABLE test;

SELECT * FROM test;

0 rows in set. Elapsed: 0.002 sec.
```

**Expected behavior**
Clickhouse would flush parts on disk.
}
        addPartContributionToDataVolume(part);
    };
    std::mutex part_select_mutex;
    try
    {
        for (size_t thread = 0; thread < num_threads; ++thread)
        {
            pool.scheduleOrThrowOnError([&, thread]
            {
                while (true)
                {
                    std::pair<String, DiskPtr> thread_part;
                    {
                        const std::lock_guard lock{part_select_mutex};
                        if (remaining_thread_parts.empty())
                            return;
                        
                        auto thread_idx = thread;
                        if (threads_parts[thread].empty())
                        {
                            
                            std::uniform_int_distribution<size_t> distribution(0, remaining_thread_parts.size() - 1);
                            auto it = remaining_thread_parts.begin();
                            std::advance(it, distribution(thread_local_rng));
                            thread_idx = *it;
                        }
                        auto & thread_parts = threads_parts[thread_idx];
                        thread_part = thread_parts.back();
                        thread_parts.pop_back();