[20150309]热备份与redo.txt

-- 最近一段时间看关于备份的书籍，提到热备份期间，如果对某块做dml操作，redo 日志里面是包含整个数据库，防止出现块分裂。
-- http://blog.itpub.net/267265/viewspace-1441552/

--实际上仅仅第1次会做，后续的dml就不会做这样的操作，做一个测试例子：

1.测试环境:
SCOTT@test> @ver1
PORT_STRING                    VERSION        BANNER
------------------------------ -------------- --------------------------------------------------------------------------------
x86_64/Linux 2.4.xx            11.2.0.3.0     Oracle Database 11g Enterprise Edition Release 11.2.0.3.0 - 64bit Production

SCOTT@test> alter tablespace users begin backup ;
Tablespace altered.

2.开始测试：
SCOTT@test> set autot traceonly
SCOTT@test> update dept set loc=upper(loc) ;

4 rows updated.

Execution Plan
----------------------------------------------------------
Plan hash value: 921533340

---------------------------------------------------------------------------
| Id  | Operation          | Name | Rows  | Bytes | Cost (%CPU)| Time     |
---------------------------------------------------------------------------
|   0 | UPDATE STATEMENT   |      |     4 |    32 |     3   (0)| 00:00:01 |
|   1 |  UPDATE            | DEPT |       |       |            |          |
|   2 |   TABLE ACCESS FULL| DEPT |     4 |    32 |     3   (0)| 00:00:01 |
---------------------------------------------------------------------------

Statistics
----------------------------------------------------------
          0  recursive calls
          5  db block gets
          8  consistent gets
          0  physical reads
       9664  redo size
        834  bytes sent via SQL*Net to client
        784  bytes received via SQL*Net from client
          3  SQL*Net roundtrips to/from client
          1  sorts (memory)
          0  sorts (disk)
          4  rows processed

SCOTT@test> commit ;
Commit complete.

--可以发现产生的redo size=9664.

3.如果我后续依旧做这样的操作：
SCOTT@test> update dept set loc=lower(loc) ;
4 rows updated.

Execution Plan
----------------------------------------------------------
Plan hash value: 921533340
---------------------------------------------------------------------------
| Id  | Operation          | Name | Rows  | Bytes | Cost (%CPU)| Time     |
---------------------------------------------------------------------------
|   0 | UPDATE STATEMENT   |      |     4 |    32 |     3   (0)| 00:00:01 |
|   1 |  UPDATE            | DEPT |       |       |            |          |
|   2 |   TABLE ACCESS FULL| DEPT |     4 |    32 |     3   (0)| 00:00:01 |
---------------------------------------------------------------------------
Statistics
----------------------------------------------------------
          0  recursive calls
          5  db block gets
          8  consistent gets
          0  physical reads
       1436  redo size
        837  bytes sent via SQL*Net to client
        784  bytes received via SQL*Net from client
          3  SQL*Net roundtrips to/from client
          1  sorts (memory)
          0  sorts (disk)
          4  rows processed

SCOTT@test> commit ;
Commit complete.

--可以发现产生的redo size=1436.

--再次证明，仅仅第1次产生的redo包含整个数据块，后续的修改就不需要这样了。