**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.
{
    return total_active_size_rows.load(std::memory_order_acquire);
}
size_t MergeTreeData::getPartsCount() const
{
    return total_active_size_parts.load(std::memory_order_acquire);
}
size_t MergeTreeData::getMaxPartsCountForPartitionWithState(DataPartState state) const
{
    auto lock = lockParts();
    size_t res = 0;
    size_t cur_count = 0;
    const String * cur_partition_id = nullptr;
    for (const auto & part : getDataPartsStateRange(state))
    {
        if (cur_partition_id && part->info.partition_id == *cur_partition_id)
        {
            ++cur_count;
        }
        else
        {
            cur_partition_id = &part->info.partition_id;
            cur_count = 1;
        }
        res = std::max(res, cur_count);
    }
    return res;
}
size_t MergeTreeData::getMaxPartsCountForPartition() const