Oracle redo 与 undo
Oracle数据库中最重要的两部分数据：redo与undo。redo(重做信息)是oracle 在线（或归档）重做日志文件中记录的信息，万一出现失效时可以利用这些数据来“重放”（或重做）事务。
undo(撤销信息)是Oracle在undo段中记录的信息，用于取消或回滚事务。
1 什么是redo
重做日志文件（redo log file）对Oracle数据库来说至关重要。Oracle维护着两类重做日志文件：
在线（online）重做日志文件和归档（archived）重做日志文件。 都用于恢复。
其主要目的是：万一实例失效或者介质失败，它们就能派上用场。
归档重做日志文件实际上就是已填满的”旧“在线重做日志文件的副本。
2 什么是undo
对数据库执行修改时，数据库会生成undo信息。以便回到更改前的状态。
redo用于在失败时重放事务（恢复事务），undo则用于取消一条语句或一组语句的作用。
与redo不同，undo在数据库内部存储在一组特殊的段中，这称为undo段（undo segment）
怎么才能看到undo生成（undo generation)的具体情况呢？
（1）创建一个空表
（2）对它做一个全表扫描，观察读表所执行的I/O数量。
（3）在表中填入许多行（但没有提交）
（4）回滚这个工作、并撤销。
（5）再次进行全表扫描，观察所执行的I/O数量。
scott@orclpdb1:orclcdb> create table t
2 as
3 select *
4 from all_objects
5 where 1=0;
Table created.
scott@orclpdb1:orclcdb> set autotrace on
scott@orclpdb1:orclcdb> select * from t;
no rows selected
Execution Plan
----------------------------------------------------------
Plan hash value: 1601196873
--------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
--------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | 481 | 2 (0)| 00:00:01 |
| 1 | TABLE ACCESS FULL| T | 1 | 481 | 2 (0)| 00:00:01 |
--------------------------------------------------------------------------
Statistics
----------------------------------------------------------
96 recursive calls
181 db block gets
148 consistent gets
1 physical reads
30504 redo size
2375 bytes sent via SQL*Net to client
366 bytes received via SQL*Net from client
1 SQL*Net roundtrips to/from client
2 sorts (memory)
0 sorts (disk)
0 rows processed
scott@orclpdb1:orclcdb> set autotrace traceonly statistics
scott@orclpdb1:orclcdb> select * from t;
no rows selected
Statistics
----------------------------------------------------------
0 recursive calls
0 db block gets
0 consistent gets
0 physical reads
0 redo size
2375 bytes sent via SQL*Net to client
366 bytes received via SQL*Net from client
1 SQL*Net roundtrips to/from client
0 sorts (memory)
0 sorts (disk)
0 rows processed
scott@orclpdb1:orclcdb>
scott@orclpdb1:orclcdb> insert into t select * from all_objects;
54936 rows created.
Statistics
----------------------------------------------------------
1377 recursive calls
9467 db block gets
105040 consistent gets
304 physical reads
9009588 redo size
494 bytes sent via SQL*Net to client
937 bytes received via SQL*Net from client
2 SQL*Net roundtrips to/from client
31 sorts (memory)
0 sorts (disk)
54936 rows processed
scott@orclpdb1:orclcdb> rollback;
Rollback complete.
scott@orclpdb1:orclcdb> select * from t;
no rows selected
Statistics
----------------------------------------------------------
2 recursive calls
1 db block gets
1134 consistent gets
0 physical reads
140 redo size
2375 bytes sent via SQL*Net to client
370 bytes received via SQL*Net from client
1 SQL*Net roundtrips to/from client
0 sorts (memory)
0 sorts (disk)
0 rows processed
scott@orclpdb1:orclcdb> set autotrace traceonly statistics
scott@orclpdb1:orclcdb> select * from t;
no rows selected
Statistics
----------------------------------------------------------
0 recursive calls
0 db block gets
1118 consistent gets
0 physical reads
0 redo size
2375 bytes sent via SQL*Net to client
370 bytes received via SQL*Net from client
1 SQL*Net roundtrips to/from client
0 sorts (memory)
0 sorts (disk)
0 rows processed
scott@orclpdb1:orclcdb>
scott@orclpdb1:orclcdb>
scott@orclpdb1:orclcdb> create table t (x int)
2 ;
create table t (x int)
*
ERROR at line 1:
ORA-00955: name is already used by an existing object
scott@orclpdb1:orclcdb> drop table t;
Table dropped.
scott@orclpdb1:orclcdb> create table t(x int);
Table created.
scott@orclpdb1:orclcdb> select extent_id, bytes, blocks
2 from user_extents
3 where segment_name='T'
4 order by extend_id;
order by extend_id
*
ERROR at line 4:
ORA-00904: "EXTEND_ID": invalid identifier
scott@orclpdb1:orclcdb> select extent_id, bytes, blocks
2 from user_extents
3 where segment_name='T'
4 order by extent_id;
no rows selected
Statistics
----------------------------------------------------------
252 recursive calls
0 db block gets
497 consistent gets
6 physical reads
0 redo size
504 bytes sent via SQL*Net to client
446 bytes received via SQL*Net from client
1 SQL*Net roundtrips to/from client
14 sorts (memory)
0 sorts (disk)
0 rows processed
scott@orclpdb1:orclcdb> insert into t(x) values (1);
1 row created.
Statistics
----------------------------------------------------------
39 recursive calls
62 db block gets
35 consistent gets
1 physical reads
6536 redo size
494 bytes sent via SQL*Net to client
925 bytes received via SQL*Net from client
2 SQL*Net roundtrips to/from client
1 sorts (memory)
0 sorts (disk)
1 rows processed
scott@orclpdb1:orclcdb> rollback;
Rollback complete.
scott@orclpdb1:orclcdb> select extent_id, bytes, blocks
2 from user_extents
3 where segment_name='T'
4 order by extent_id;
1 row selected.
Statistics
----------------------------------------------------------
1 recursive calls
0 db block gets
135 consistent gets
0 physical reads
0 redo size
709 bytes sent via SQL*Net to client
457 bytes received via SQL*Net from client
2 SQL*Net roundtrips to/from client
1 sorts (memory)
0 sorts (disk)
1 rows processed
scott@orclpdb1:orclcdb> set trace off
SP2-0158: unknown SET option "trace"
scott@orclpdb1:orclcdb> set autotrace off
scott@orclpdb1:orclcdb> select extent_id, bytes, blocks
2 from user_extents
3 where segment_name='T'
4 order by extent_id;
EXTENT_ID BYTES BLOCKS
---------- ---------- ----------
0 65536 8
1 row selected.
scott@orclpdb1:orclcdb> 可以看到，表创建之后没有分配任何存储空间——这个表没有使用任何区段。完成一个INSERT 后，紧接着执行ROLLBACK，可以看到INSERT分配了存储空间，不过ROLLBACK并没有将分配的存储空间”释放“。
总结：
- 段确实由INSERT创建但是未被ROLLBACK撤销。
- 由INSERT新创建的格式化块被第二次扫描，这两方面结合在一起，说明回滚只是一种逻辑上”将数据库恢复原状态“的操作。数据库并不会完全恢复原状，只是逻辑上相同而已。
3. redo 和undo如何协作
尽管undo信息存储在undo表空间或undo段中，但也会受到redo的保护。
意思是，会把undo数据当成是表数据或索引数据已有，对undo的修改会生成一些redo,这些redo将计入日志。将undo数据增加到undo段中，并像其他部分的数据一样，在缓冲区中得到缓存。
提交和回滚处理
4.提交和回滚处理
COMMIT做什么
Rollback做什么
5.分析redo
生成的redo越多，操作花费的时间就越长，整个系统也会越慢。
能关掉重做日志生成程序吗？
不能，因为重做日志对数据库至关重要，它不是开销，不是浪费。重做日志是实实在在必不可少的。是数据库采用的工作方式。 如果你关闭了redo,那么磁盘驱动器的任何暂时失败，掉电或某个软件崩溃都会导致整个数据库不可用。而且不可恢复。