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.

return command;
    }
    else if (command_ast->type == ASTAlterCommand::DROP_CONSTRAINT)
    {
        AlterCommand command;
        command.ast = command_ast->clone();
        command.if_exists = command_ast->if_exists;
        command.type = AlterCommand::DROP_CONSTRAINT;
        command.constraint_name = command_ast->constraint->as<ASTIdentifier &>().name;
        return command;
    }
    else if (command_ast->type == ASTAlterCommand::DROP_INDEX)
    {
        AlterCommand command;
        command.ast = command_ast->clone();
        command.type = AlterCommand::DROP_INDEX;
        command.index_name = command_ast->index->as<ASTIdentifier &>().name;
        command.if_exists = command_ast->if_exists;
        if (command_ast->clear_index)
            command.clear = true;
        if (command_ast->partition)
            command.partition = command_ast->partition;
        return command;
    }
    else if (command_ast->type == ASTAlterCommand::MODIFY_TTL)
    {
        AlterCommand command;
        command.ast = command_ast->clone();
        command.type = AlterCommand::MODIFY_TTL;
        command.ttl = command_ast->ttl;