I tried to rename a key column in a MergeTree table. It does not work, and 20.4 is very clear about the reason. 
20.5.2 is complaining about non-existing column with the old name, suggesting there is a column with the new name:
```
ALTER TABLE rename_key
    RENAME COLUMN key TO new

Received exception from server (version 20.4.6):
Code: 44. DB::Exception: Received from localhost:9000. 
DB::Exception: Trying to ALTER RENAME key key column which is a part of key expression.

Received exception from server (version 20.5.2):
Code: 47. DB::Exception: Received from localhost:9000. 
DB::Exception: Missing columns: 'key' while processing query: 'key', required columns: 'key', source columns: 'value' 'new' 'date'.
```
The message hasn't changed for the partition key:
```
ALTER TABLE rename_key
    RENAME COLUMN date TO new

Received exception from server (version 20.4.6):
Code: 44. DB::Exception: Received from localhost:9000. 
DB::Exception: Trying to ALTER RENAME key date column which is a part of key expression.

Received exception from server (version 20.5.2):
Code: 524. DB::Exception: Received from localhost:9000. 
DB::Exception: Trying to ALTER RENAME key date column which is a part of key expression.
```
The table:
```
CREATE TABLE rename_key
(
    `date` Date,
    `key` UInt64,
    `value` String
)
ENGINE = MergeTree()
PARTITION BY date
ORDER BY key
```

}
        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))