Solution : https://service.sap.com/sap/support/notes/1088403 (SAP Service marketplace login required)
Key words : 
update table set col = space, open sql select statement, relevant select statement includes, type-specific initial values, open sql statement, select statement mentioned, process select statements, abap runtime system, select statement returns, select statement directly
Related Notes : 
       
| 1494338 | |
| 658322 | Composite SAP Note: Incorrect resulting set with Oracle |