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.type == AlterCommand::MODIFY_COLUMN)
        {
            if (!all_columns.has(column_name))
            {
                if (!command.if_exists)
                    throw Exception{"Wrong column name. Cannot find column " + backQuote(column_name) + " to modify",
                                    ErrorCodes::NOT_FOUND_COLUMN_IN_BLOCK};
                else
                    continue;
            }
            if (renamed_columns.count(column_name))
                throw Exception{"Cannot rename and modify the same column " + backQuote(column_name) + " in a single ALTER query",
                                ErrorCodes::NOT_IMPLEMENTED};
            modified_columns.emplace(column_name);
        }
        else if (command.type == AlterCommand::DROP_COLUMN)
        {
            if (all_columns.has(command.column_name) || all_columns.hasNested(command.column_name))
            {
                for (const ColumnDescription & column : all_columns)
                {
                    const auto & default_expression = column.default_desc.expression;
                    if (default_expression)
                    {
                        ASTPtr query = default_expression->clone();
                        auto syntax_result = SyntaxAnalyzer(context).analyze(query, all_columns.getAll());
                        const auto actions = ExpressionAnalyzer(query, syntax_result, context).getActions(true);
                        const auto required_columns = actions->getRequiredColumns();
                        if (required_columns.end() != std::find(required_columns.begin(), required_columns.end(), command.column_name))