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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虽然学习Java很久了，自己也连接过一些数据库，比如mysql之类的，如今呢，也学习了一段时间的Oracle，然而呢，今天是我第一次连接Oracle，嘿嘿，应该还不算太迟吧。
今天呢，有点笨拙，大家莫笑！
我这是一个查询例子
首先，� ......
1.OracleDBConsoleorcl oem控制台的服务进程
2.OracleJobSchedulerORCL 定时器的服务进程
3.OracleOraDb10g_home1iSQL*Plus isql*plus的服务进程
4.OracleOraDb10g_home1TNSListener 监听器的服务进程
5.Or ......
ORACLE中数据字典视图分为3大类, 用前缀区别，分别为：USER，ALL 和 DBA，许多数据字典视图包含相似的信息。
USER_*:有关用户所拥有的对象信息，即用户自己创建的对象信息
ALL_*：有关用户可以访问的对象的信息，即用户自己创建的对象的信息加上其他用户创建的对象但该用户有权访问的信息
DBA_* ......
查询用户信息
SELECT USERNAME,DEFAULT_TABLESPACE, TEMPORARY_TABLESPACE, PROFILE, ACCOUNT_STATUS, CREATED from dba_users; 查询用户空间使用和上限情况
SELECT username, tablespace_name, bytes/1024/1024 space_used_in_mb, max_bytes/1024/1024 max_space_in_mb from dba_ts_quotas; 创建 ......
如果你要把Oracle里的大量数据(80M以上)转移到另外的用户，另外的表空间里。可以用下面介绍的快速转移数据的方法。
一、建新表的方式
create table target_tablename tablespace
target_tablespace_name nologging
pctfree 10 pctused 60
storage(initial 5M next 5M minextents 1
maxextents unlimited pctincr ......
