Newer versions can't clear columns which are used in default expressions for other columns. Maybe it's caused by https://github.com/ClickHouse/ClickHouse/issues/6913

CH version 20.5.2


create database Temp;
create table Temp.Test (`impression_id` String,`impression_id_compressed` FixedString(16) DEFAULT UUIDStringToNum(substring(impression_id, 1, 36)), `impression_id_hashed` UInt16 DEFAULT reinterpretAsUInt16(impression_id_compressed), `event_date` Date ) ENGINE = MergeTree(event_date, impression_id_hashed, (event_date, impression_id_hashed), 8192);
alter table Temp.Test clear column `impression_id` in partition '202001';


Received exception from server (version 20.5.2):
Code: 44. DB::Exception: Received from ch7r1.local:9000. DB::Exception: Cannot drop column `impression_id`, because column `impression_id_compressed` depends on it. 




Older versions (tested with 19.17.4 and 20.3.5):

CREATE DATABASE Temp;
create table Temp.Test (`impression_id` String,`impression_id_compressed` FixedString(16) DEFAULT UUIDStringToNum(substring(impression_id, 1, 36)), `impression_id_hashed` UInt16 DEFAULT reinterpretAsUInt16(impression_id_compressed), `event_date` Date ) ENGINE = MergeTree(event_date, impression_id_hashed, (event_date, impression_id_hashed), 8192);
alter table Temp.Test clear column `impression_id` in partition '202001';

ALTER TABLE Temp.Test
    CLEAR COLUMN impression_id     IN PARTITION '202001'


Ok.

{
    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, const Context & 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;
}
}