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_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;