urowid 是"universal rowids"的意思。
rowid是物理的地址,urowid是邏輯的地址。
Oracle uses "universal rowids" (urowids) to store the addresses of index-organized
and foreign tables. Index-organized tables have logical urowids and foreign tables
have foreign urowids. Both types of urowid are stored in the ROWID pseudocolumn
(as are the physical rowids of heap-organized tables).