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.

else if (command_ast->type == ASTAlterCommand::MODIFY_ORDER_BY)
    {
        AlterCommand command;
        command.ast = command_ast->clone();
        command.type = AlterCommand::MODIFY_ORDER_BY;
        command.order_by = command_ast->order_by;
        return command;
    }
    else if (command_ast->type == ASTAlterCommand::ADD_INDEX)
    {
        AlterCommand command;
        command.ast = command_ast->clone();
        command.index_decl = command_ast->index_decl;
        command.type = AlterCommand::ADD_INDEX;
        const auto & ast_index_decl = command_ast->index_decl->as<ASTIndexDeclaration &>();
        command.index_name = ast_index_decl.name;
        if (command_ast->index)
            command.after_index_name = command_ast->index->as<ASTIdentifier &>().name;
        command.if_not_exists = command_ast->if_not_exists;
        return command;
    }
    else if (command_ast->type == ASTAlterCommand::ADD_CONSTRAINT)
    {
        AlterCommand command;
        command.ast = command_ast->clone();
        command.constraint_decl = command_ast->constraint_decl;
        command.type = AlterCommand::ADD_CONSTRAINT;
        const auto & ast_constraint_decl = command_ast->constraint_decl->as<ASTConstraintDeclaration &>();
        command.constraint_name = ast_constraint_decl.name;
        command.if_not_exists = command_ast->if_not_exists;