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
```

if (!has_column && command.if_exists)
                command.ignore = true;
        }
    }
    prepared = true;
}
void AlterCommands::validate(const StorageInMemoryMetadata & metadata, const Context & context) const
{
    auto all_columns = metadata.columns;
    
    ASTPtr default_expr_list = std::make_shared<ASTExpressionList>();
    NameSet modified_columns, renamed_columns;
    for (size_t i = 0; i < size(); ++i)
    {
        const auto & command = (*this)[i];
        const auto & column_name = command.column_name;
        if (command.type == AlterCommand::ADD_COLUMN)
        {
            if (all_columns.has(column_name) || all_columns.hasNested(column_name))
            {
                if (!command.if_not_exists)
                    throw Exception{"Cannot add column " + backQuote(column_name) + ": column with this name already exists",
                                    ErrorCodes::DUPLICATE_COLUMN};
                else
                    continue;
            }
            if (!command.data_type)
                throw Exception{"Data type have to be specified for column " + backQuote(column_name) + " to add",
                                ErrorCodes::BAD_ARGUMENTS};
            all_columns.add(ColumnDescription(column_name, command.data_type));