ORACLE 一致读(Consistent Read)的步骤
1. Read the Data Block.
2. Read the Row Header.
3. Check the Lock Byte to determine whether there's an ITL entry.
4. Read the ITL entry to determine the Transaction ID (Xid).
5. Read the Transaction Table using the Transaction ID. If the transaction has been committed and has a System Commit Number less than the query's System Change Number, update the status of the block (block cleanout) and start over at step 1.
6. Read the last undo block (Uba).
7. Compare the block transaction ID with the transaction table transaction ID. If the Transaction ID in the undo block doesn't equal the Transaction ID from the Transaction Table, then issue ORA-1555, Snapshot Too Old.
8. If the Transaction IDs are identical, make a copy of the data block in memory. Starting with the head undo entry, apply the changes to the copied data block.
9. If the tail undo entry (the actual first undo entry in the chain, or the last in the chain going backwards!) indicates another data block address, read the indicated undo block into memory and repeat steps 7 and 8 until the undo entries don't contain a value for the data block address.
10. When there's no "previous data block address," the transaction has been completely undone.
11. If the undo entry contains:
a. a pointer to a previous transaction undo block address, read the Transaction ID in the previous transaction undo block header and read the appropriate Transaction Table entry. Return to step 5.
b. an ITL record, restore the ITL record to the data block. Return to step 4.
注释说明：
事实上在第6步之前，会先比较query scn和control scn(下面红色字体是解释)，如果control scn > query scn，oracle根本就不去尝试读取undo block了，因为注定失败，立刻报1555错误.
第7步，表示
不是修改了当前block的事务没有commit
就是事务commit了但是commit scn >query scn，（因为第5步已经判断commit scn<query scn的情况）
所以准备要读取回滚段中的前镜像。
第9步，你可以把rollback的过程想象为一个链表，从一个undo block链到另一个undo block，�
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