I'm trying to create a table with a compound primary key where the second column is ordered descending:

```
CREATE TABLE AccountHistory (                                                                                                                                                                           
  AccountNumber BIGINT NOT NULL,                                                                                                                                                                        
  Ts TIMESTAMP NOT NULL,                                                                                                                                                                                
  Memo TEXT,                                                                                                                                                                                            
  ChangeAmount BIGINT NOT NULL,                                                                                                                                                                         
  PRIMARY KEY (AccountNumber, ts DESC)                                                                                                                                                              
);

```

However, PostgreSQL is saying there's a syntax error at the `DESC` clause. 

* Does PostgreSQL really not allow this?
* Does it ever make sense to have a DESC key like this?
* Is my only option to create an additional index with my desired semantics?