Solution : https://service.sap.com/sap/support/notes/738098 (SAP Service marketplace login required)
Key words : 
bi platform > olap > special olap functions, services > hierarchy > edit hierarchy > hierarchy attributes, internal hierarchy display, shared buffer reason, current nw04 documentation, suppress 'unassigned' node, experiencing performance problems, hierarchy attributes, hierarchy belongs, hierarchy maintenance
Related Notes : 
       
| 1614788 | Buffering of large hierarchies in BW reporting | 
| 693495 | SAPBWNews BW Support Package 05 NW'04 Stack 05 RIN | 
| 693494 | SAPBWNews BW SP04 NW'04 Stack 04 RIN | 
| 679667 | SAPBWNews BW 3.1 Content Support Package 16 | 
| 679666 | SAPBWNews BW 3.0B Support Package 22 | 
| 156957 | OLAP: Message 051(BRAIN) when executing a query |