20.12.5.14 `alter table ... drop part ...` removes from ZooKeeper all blocks for the partition.
The same thing happens when CH cleans empty parts:
```
CREATE TABLE empty_parts 
(
  `k`   Int8,
  `v`   Int8
)
ENGINE = ReplicatedSummingMergeTree ('/clickhouse/empty_parts','replica1')
ORDER BY k;

insert into empty_parts values (1, 15);
insert into empty_parts values (1, -15);
optimize table empty_parts;
insert into empty_parts values (2, -15);

select name, active, rows, modification_time from system.parts where table like 'empty_parts'

name        | active | rows | modification_time  
------------+--------+------+--------------------
all_15_15_0 |      0 |    1 | 2021-01-07 21:54:05
all_16_16_0 |      0 |    1 | 2021-01-08 20:13:00
all_15_16_1 |      1 |    0 | 2021-01-08 20:13:46
all_17_17_0 |      1 |    1 | 2021-01-08 20:13:47


select name, value from system.zookeeper where path='/clickhouse/empty_parts/blocks/'

name                                         | value      
---------------------------------------------+------------
all_9295045124109325217_14756667786308374723 | all_16_16_0
all_18396465411487747783_4385790938943258888 | all_17_17_0
```
The next insert of the same block is deduplicated:
```
insert into empty_parts values (2, -15);

2021.01.08 20:13:53.267392 <Information> empty_parts (Replicated OutputStream): Block with ID all_18396465411487747783_4385790938943258888 already exists locally as part all_17_17_0; ignoring it.
```
The empty part is getting dropped in background
```
2021.01.08 20:14:19.103462 <Trace> empty_parts: Will try to insert a log entry to DROP_RANGE for part: all_15_16_1
```
The next insert of the same block is NOT deduplicated:
```
insert into empty_parts values (2, -15);

2021.01.08 20:16:19.145203 <Debug> executeQuery: (from [::1]:40824) insert into empty_parts values
2021.01.08 20:16:19.146681 <Debug> empty_parts (Replicated OutputStream): Wrote block with ID 'all_18396465411487747783_4385790938943258888', 1 rows
2021.01.08 20:16:19.181508 <Trace> empty_parts: Renaming temporary part tmp_insert_all_23_23_0 to all_18_18_0.


select name, active, rows, modification_time from system.parts where table like 'empty_parts'

name        | active | rows | modification_time  
------------+--------+------+--------------------
all_17_17_0 |      1 |    1 | 2021-01-08 16:13:47
all_18_18_0 |      1 |    1 | 2021-01-08 16:16:19


select name, value from system.zookeeper where path='/clickhouse/empty_parts/blocks/'

name                                         | value      
---------------------------------------------+------------
all_18396465411487747783_4385790938943258888 | all_18_18_0

SELECT _part, * FROM empty_parts

┌─_part───────┬─k─┬──v─┐
│ all_17_17_0 │ 1 │ 15 │
└─────────────┴───┴────┘
┌─_part───────┬─k─┬──v─┐
│ all_18_18_0 │ 1 │ 15 │
└─────────────┴───┴────┘
```

/** Let's check that there is no such partition in the `detached` directory (where we will write the downloaded parts).
      * Unreliable (there is a race condition) - such a partition may appear a little later.
      */
    Poco::DirectoryIterator dir_end;
    for (const std::string & path : getDataPaths())
    {
        for (Poco::DirectoryIterator dir_it{path + "detached/"}; dir_it != dir_end; ++dir_it)
        {
            MergeTreePartInfo part_info;
            if (MergeTreePartInfo::tryParsePartName(dir_it.name(), &part_info, format_version)
                && part_info.partition_id == partition_id)
                throw Exception("Detached partition " + partition_id + " already exists.", ErrorCodes::PARTITION_ALREADY_EXISTS);
        }
    }
    zkutil::Strings replicas;
    zkutil::Strings active_replicas;
    String best_replica;
    {
        
        replicas = zookeeper->getChildren(from + "/replicas");
        
        active_replicas.reserve(replicas.size());
        for (const String & replica : replicas)
            if (zookeeper->exists(from + "/replicas/" + replica + "/is_active"))
                active_replicas.push_back(replica);
        if (active_replicas.empty())
            throw Exception("No active replicas for shard " + from, ErrorCodes::NO_ACTIVE_REPLICAS);
        /** You must select the best (most relevant) replica.
        * This is a replica with the maximum `log_pointer`, then with the minimum `queue` size.
        * NOTE This is not exactly the best criteria. It does not make sense to download old partitions,