22.2.1.4422.

```
create table test_alias (K Int64, D Date, S String, B Int64 Alias length(S)) Engine=MergeTree order by K;
insert into test_alias select number, today(), 'sdfsdfsdfsdfsdfds' from numbers(10000000);

session1: 
clickhouse-benchmark -c 10 <<< "insert into test_alias select number, today(), 'sdfsdfsdfsdfsdfds' from numbers(10000)"

session2: 
alter table test_alias drop column B;
0 rows in set. Elapsed: 251.043 sec.
```

especially it can be a problem with S3 disks

Alias columns do not persist in parts even in columns.txt
{
    MutationCommand command;
    auto ast = std::make_shared<ASTAlterCommand>();
    ast->type = ASTAlterCommand::MATERIALIZE_TTL;
    command.type = MutationCommand::MATERIALIZE_TTL;
    command.ast = std::move(ast);
    return command;
}
MutationCommands AlterCommands::getMutationCommands(StorageInMemoryMetadata metadata, bool materialize_ttl, ContextPtr context) const
{
    MutationCommands result;
    for (const auto & alter_cmd : *this)
        if (auto mutation_cmd = alter_cmd.tryConvertToMutationCommand(metadata, context); mutation_cmd)
            result.push_back(*mutation_cmd);
    if (materialize_ttl)
    {
        for (const auto & alter_cmd : *this)
        {
            if (alter_cmd.isTTLAlter(metadata))
            {
                result.push_back(createMaterializeTTLCommand());
                break;
            }
        }
    }
    return result;
}
}