MERGE SUBPARTITIONS
   subpartition | { FOR ( subpartition_key_value [, subpartition_key_value ]... ) },
   subpartition | { FOR ( subpartition_key_value [, subpartition_key_value ]... ) }
   [ INTO { range_subpartition_desc 
          | list_subpartition_desc
          }
   ]
   [ dependent_tables_clause ]
   [ update_index_clauses ]
   [ parallel_clause ]