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
```

{
            
            
            
            primary_key = KeyDescription::getKeyFromAST(sorting_key.definition_ast, metadata.columns, context);
        }
        
        sorting_key.recalculateWithNewAST(order_by, metadata.columns, context);
    }
    else if (type == COMMENT_COLUMN)
    {
        metadata.columns.modify(column_name,
            [&](ColumnDescription & column) { column.comment = *comment; });
    }
    else if (type == ADD_INDEX)
    {
        if (std::any_of(
                metadata.secondary_indices.cbegin(),
                metadata.secondary_indices.cend(),
                [this](const auto & index)
                {
                    return index.name == index_name;
                }))
        {
            if (if_not_exists)
                return;
            else
                throw Exception{"Cannot add index " + index_name + ": index with this name already exists",
                                ErrorCodes::ILLEGAL_COLUMN};
        }