RDBMS如何处理表更新?

假设我有一个表和一个索引

original simple table A
------------------------
rowid  |  id   name
123    |  1    A
124    |  4    G
125    |  2    R
126    |  3    P

index on A.id
-------------
id  rowid
1   123
2   125
3   126
4   124

此时,我执行这个DML语句

UPDATE A SET id = 5 WHERE id = 4

执行此语句时会发生什么?

a)

BEGIN
go to index
search for `id == 4` (B tree index generally)
find that `rowid = 124`
go to that location
update id in the table
come back (? I am not sure)
update the index
END

b)

BEGIN
go to index
search for `id == 4` (B tree index generally)
update the id value in index
find that `rowid = 124`
go to that location
update id in the table
END

c)其他事情完全发生了

因为这可能依赖于数据库本身, 在甲骨文中是如何发生的?