Oracle 10g 闪回查询特性的增强：
Oracle 9i提供了闪回特性增强，为数据恢复带来了极大的方便，但是Oracle 9i的闪回查询只能提供某个时间点的数据视图，并不能告诉用户这样的数据经过了几个事务、怎样的修改（UPDATE、INSERT、DELETE等），而这些信息在回滚段中是存在的，在Oracle 10g中，Oracle进一步加强了闪回查询的特性，提供了以下两种闪回查询：
·闪回版本查询（Flashback Versions Query）；
·闪回事务查询（Flashback Transaction Query）。
闪回版本查询允许使用一个新的VERSIONS子句查询两个时间点或者SCN之间的数据版本。这些版本可以按照事务进行区分，闪回版本查询只返回提交数据，未提交数据不被显示。
通过以下示例，来理解闪回版本查询的信用。首先创建一个测试表，执行一系列的DML操作：
tq@TQGZS> create table t as select username,user_id from dba_users;
Table created.
tq@TQGZS> select * from t;USERNAME USER_ID
------------------------------ ----------
TQ 76
RMAN 71
T2 69
PERFSTAT 62
TEST 66
HR 55
OE 56
DIP 19
SH 58
IX 57
MDDATA 50
PM 59
T4 72
BI 60
T5 73
SQLLDR 75
T1 68
T6 74
TSMSYS 21
T3 70
SCOTT 54
OLAPSYS 47
SI_INFORMTN_SCHEMA 45
ORDPLUGINS 44
XDB 38
SYSMAN 51
ANONYMOUS 39
CTXSYS 36
WMSYS 25
DBSNMP 24
DMSYS 35
EXFSYS 34
ORDSYS 43
MDSYS 46
SYSTEM 5
SYS 0
MGMT_VIEW 53
CSMIG 64
OUTLN 11
39 rows selected.
tq@TQGZS> delete from t where username='OUTLN';
1 row deleted.
tq@TQGZS> commit;
Commit complete.
tq@TQGZS> delete from t where username='TEST';
1 row deleted.
tq@TQGZS> commit;
Commit complete.
tq@TQGZS> update t set user_id = 6 where username = 'TQ';
1 row updated.
tq@TQGZS> commit;
Commit complete.
tq@TQGZS> delete from t where user_id > 10;
34 rows deleted.
tq@TQGZS> commit;
Commit complete.
tq@TQGZS> select * from t;
USERNAME USER_ID
-------------------- ----------
TQ 6
SYSTEM 5
SYS 0
tq@TQGZS> insert into t values('DBTAN',8);
1 row created.
tq@TQGZS> commit;
Commit complete.
至此数据库中已经交替执行了多个事务，进行了众多的数据修改，现在的测试表已经与最初完全不同了。如果使用Oracle 9i的闪回查询，是很难区分这些不同事务的变更，找到合适的、正确的数据将变得极为困难。
再了看看Oracle 10g的闪回版本查询，通过vertions子句和对数据表引入了系列的伪列（versions_starttime等），可以获得对数据表的所有事务操作，注意以下输出中versions_operation代表了不同类型的操作（D-Delete、I-Insert、U-Update），VERSIONS_XID是一个重要数据，代表了不同版本的事务ID：
tq@TQGZS> select versions_starttime,versions_endtime,versions_xid,
2 versions_operation,username,user_id
3 from t versions between timestamp minvalue and maxvalue;VERSIONS_STARTTIME VERSIONS_ENDTIME VERSIONS_XID V USERNAME USER_ID
------------------------- ------------------------- ---------------- - -------------------- -------
04-JAN-10 11.28.50 AM 0300280024190000 D CSMIG 64
04-JAN-10 11.28.50 AM 0300280024190000 D MGMT_VIEW 53
04-JAN-10 11.28.50 AM 0300280024190000 D MDSYS 46
04-JAN-10 11.28.50 AM 0300280024190000 D ORDSYS 43
04-JAN-10 11.28.50 AM 0300280024190000 D EXFSYS 34
04-JAN-10 11.28.50 AM 0300280024190000 D DMSYS 35
04-JAN-10 11.28.50 AM 0300280024190000 D DBSNMP 24
04-JAN-10 11.28.50 AM 0300280024190000 D WMSYS 25
04-JAN-10 11.28.50 AM 0300280024190000 D CTXSYS 36
04-JAN-10 11.28.50 AM 0300280024190000 D ANONYMOUS 39
04-JAN-10 11.28.50 AM 0300280024190000 D SYSMAN 51
04-JAN-10 11.28.50 AM 0300280024190000 D XDB 38
04-JAN-10 11.28.50 AM 0300280024190000 D ORDPLUGINS 44
04-JAN-10 11.28.50 AM 0300280024190000 D SI_INFORMTN_SCHEMA 45
04-JAN-10 11.28.50 AM 0300280024190000 D OLAPSYS 47
04-JAN-10 11.28.50 AM 0300280024190000 D SCOTT 54
04-JAN-10 11.28.50 AM 0300280024190000 D T3 70
04-JAN-10 11.28.50 AM 0300280024190000 D TSMSYS 21
04-JAN-10 11.28.50 AM 0300280024190000 D T6 74
04-JAN-10 11.28.50 AM 0300280024190000 D T1 68
04-JAN-10 11.28.50 AM 0300280024190000 D SQLLDR 75
04-JAN-10 11.28.50 AM 0300280024190000 D T5 73
04-JAN-10 11.28.50 AM 0300280024190000 D BI 60
04-JAN-10 11.28.50 AM 0300280024190000 D T4 72
04-JAN-10 11.28.50 AM 0300280024190000 D PM 59
04-JAN-10 11.28.50 AM 0300280024190000 D MDDATA 50
04-JAN-10 11.28.50 AM 0300280024190000 D IX 57
04-JAN-10 11.28.50 AM 0300280024190000 D SH 58
04-JAN-10 11.28.50 AM 0300280024190000 D DIP 19
04-JAN-10 11.28.50 AM 0300280024190000 D OE 56
04-JAN-10 11.28.50 AM 0300280024190000 D HR 55
04-JAN-10 11.28.50 AM 0300280024190000 D PERFSTAT 62
04-JAN-10 11.28.50 AM 0300280024190000 D T2 69
04-JAN-10 11.28.50 AM 0300280024190000 D RMAN 71
04-JAN-10 11.28.29 AM 0700190084200000 U TQ 6
04-JAN-10 11.28.29 AM TQ 76
04-JAN-10 11.28.50 AM RMAN 71
04-JAN-10 11.28.50 AM T2 69
04-JAN-10 11.28.50 AM PERFSTAT 62
04-JAN-10 11.28.50 AM HR 55
04-JAN-10 11.28.50 AM OE 56
04-JAN-10 11.28.50 AM DIP 19
04-JAN-10 11.28.50 AM SH 58
04-JAN-10 11.28.50 AM IX 57
04-JAN-10 11.28.50 AM MDDATA 50
04-JAN-10 11.28.50 AM PM 59
04-JAN-10 11.28.50 AM T4 72
04-JAN-10 11.28.50 AM BI 60
04-JAN-10 11.28.50 AM T5 73
04-JAN-10 11.28.50 AM SQLLDR 75
04-JAN-10 11.28.50 AM T1 68
04-JAN-10 11.28.50 AM T6 74
04-JAN-10 11.28.50 AM TSMSYS 21
04-JAN-10 11.28.50 AM T3 70
04-JAN-10 11.28.50 AM SCOTT 54
04-JAN-10 11.28.50 AM OLAPSYS 47
04-JAN-10 11.28.50 AM SI_INFORMTN_SCHEMA 45
04-JAN-10 11.28.50 AM ORDPLUGINS 44
04-JAN-10 11.28.50 AM XDB 38
04-JAN-10 11.28.50 AM SYSMAN 51
04-JAN-10 11.28.50 AM ANONYMOUS 39
04-JAN-10 11.28.50 AM CTXSYS 36
04-JAN-10 11.28.50 AM WMSYS 25
04-JAN-10 11.28.50 AM DBSNMP 24
04-JAN-10 11.28.50 AM DMSYS 35
04-JAN-10 11.28.50 AM EXFSYS 34
04-JAN-10 11.28.50 AM ORDSYS 43
04-JAN-10 11.28.50 AM MDSYS 46
SYSTEM 5
SYS 0
04-JAN-10 11.28.50 AM MGMT_VIEW 53
04-JAN-10 11.28.50 AM CSMIG 64
04-JAN-10 11.29.35 AM 07001D0084200000 I DBTAN 8
73 rows selected.
通过以上输出，根据VERSIONS_XID可以清晰地区分不同事务在不同时间对数据所做的更改。具备了flashback versions query查询的基础，就可以进行基于flashback versions query的事务级恢复，这就是flashback transaction query。
flashback transaction query可以从FLASHBACK_TRANSACTION_QUERY视图中获得指定事务级的历史信息以及Undo_SQL，通过这个UNDO_SQL，就可以撤销特定的提交事务。
flashback transaction query需要用到FLASHBACK_TRANSACTION_QUERY视图，先看一下视图：
sys@TQGZS> desc flashback_transaction_query;
Name Null? Type
----------------------- -------- ------------------------
XID RAW(8)
START_SCN NUMBER
START_TIMESTAMP DATE
COMMIT_SCN NUMBER
COMMIT_TIMESTAMP DATE
LOGON_USER VARCHAR2(30)
UNDO_CHANGE# NUMBER
OPERATION VARCHAR2(32)
TABLE_NAME VARCHAR2(256)
TABLE_OWNER VARCHAR2(32)
ROW_ID VARCHAR2(19)
UNDO_SQL VARCHAR2(4000)
该视图的定义为：
sys@TQGZS> select dbms_metadata.get_ddl('VIEW','FLASHBACK_TRANSACTION_QUERY') from dual;
DBMS_METADATA.GET_DDL('VIEW','FLASHBACK_TRANSACTION_QUERY')
--------------------------------------------------------------------------------CREATE OR REPLACE FORCE VIEW "SYS"."FLASHBACK_TRANSACTION_QUERY" ("XID", "STAR
T_SCN", "START_TIMESTAMP", "COMMIT_SCN", "COMMIT_TIMESTAMP", "LOGON_USER", "UNDO
_CHANGE#", "OPERATION", "TABLE_NAME", "TABLE_OWNER", "ROW_ID", "UNDO_SQL") AS
select xid, start_scn, start_timestamp,
decode(commit_scn, 0, commit_scn, 281474976710655, NULL, commit_scn)
commit_scn, commit_timestamp,
logon_user, undo_change#, operation, table_name, table_owner,
row_id, undo_sql
from sys.x$ktuqqry
继续前面的测试，如果需要撤销XID＝0300280024190000的事务，可以通过如下步骤进行（注意当查询FLASHBACK_TRANSACTION_QUERY视图时如果直接引用XID则查询会因为无法使用索引而极其耗时）：
tq@TQGZS> set timing on
tq@TQGZS> set autotrace on
tq@TQGZS> select undo_sql from flashback_transaction_query
2 where xid = '0300280024190000';
UNDO_SQL
--------------------------------------------------------------------------------
insert into "TQ"."T"("USERNAME","USER_ID") values ('CSMIG','64');
insert into "TQ"."T"("USERNAME","USER_ID") values ('MGMT_VIEW','53');
insert into "TQ"."T"("USERNAME","USER_ID") values ('MDSYS','46');
insert into "TQ"."T"("USERNAME","USER_ID") values ('ORDSYS','43');
insert into "TQ"."T"("USERNAME","USER_ID") values ('EXFSYS','34');
insert into "TQ"."T"("USERNAME","USER_ID") values ('DMSYS','35');
insert into "TQ"."T"("USERNAME","USER_ID") values ('DBSNMP','24');
insert into "TQ"."T"("USERNAME","USER_ID") values ('WMSYS','25');
insert into "TQ"."T"("USERNAME","USER_ID") values ('CTXSYS','36');
insert into "TQ"."T"("USERNAME","USER_ID") values ('ANONYMOUS','39');
insert into "TQ"."T"("USERNAME","USER_ID") values ('SYSMAN','51');
insert into "TQ"."T"("USERNAME","USER_ID") values ('XDB','38');
insert into "TQ"."T"("USERNAME","USER_ID") values ('ORDPLUGINS','44');
insert into "TQ"."T"("USERNAME","USER_ID") values ('SI_INFORMTN_SCHEMA','45');
insert into "TQ"."T"("USERNAME","USER_ID") values ('OLAPSYS','47');
insert into "TQ"."T"("USERNAME","USER_ID") values ('SCOTT','54');
insert into "TQ"."T"("USERNAME","USER_ID") values ('T3','70');
insert into "TQ"."T"("USERNAME","USER_ID") values ('TSMSYS','21');
insert into "TQ"."T"("USERNAME","USER_ID") values ('T6','74');
insert into "TQ"."T"("USERNAME","USER_ID") values ('T1','68');
insert into "TQ"."T"("USERNAME","USER_ID") values ('SQLLDR','75');
insert into "TQ"."T"("USERNAME","USER_ID") values ('T5','73');
insert into "TQ"."T"("USERNAME","USER_ID") values ('BI','60');
insert into "TQ"."T"("USERNAME","USER_ID") values ('T4','72');
insert into "TQ"."T"("USERNAME","USER_ID") values ('PM','59');
insert into "TQ"."T"("USERNAME","USER_ID") values ('MDDATA','50');
insert into "TQ"."T"("USERNAME","USER_ID") values ('IX','57');
insert into "TQ"."T"("USERNAME","USER_ID") values ('SH','58');
insert into "TQ"."T"("USERNAME","USER_ID") values ('DIP','19');
insert into "TQ"."T"("USERNAME","USER_ID") values ('OE','56');
insert into "TQ"."T"("USERNAME","USER_ID") values ('HR','55');
insert into "TQ"."T"("USERNAME","USER_ID") values ('PERFSTAT','62');
insert into "TQ"."T"("USERNAME","USER_ID") values ('T2','69');
insert into "TQ"."T"("USERNAME","USER_ID") values ('RMAN','71');
35 rows selected.
Elapsed: 00:00:22.71
Execution Plan
----------------------------------------------------------
Plan hash value: 1115820779
--------------------------------------
| Id | Operation | Name |
--------------------------------------
| 0 | SELECT STATEMENT | |
|* 1 | FIXED TABLE FULL| X$KTUQQRY |
--------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
1 - filter(RAWTOHEX("XID")='0300280024190000')
Note
-----
- rule based optimizer used (consider using cbo)
Statistics
----------------------------------------------------------
28346 recursive calls
0 db block gets
1082754 consistent gets
2 physical reads
0 redo size
3036 bytes sent via SQL*Net to client
407 bytes received via SQL*Net from client
4 SQL*Net roundtrips to/from client
255 sorts (memory)
0 sorts (disk)
35 rows processed
此时可以通过hextoraw转换利用底层索引，提高查询速度：
tq@TQGZS> set autotrace traceonly explain
tq@TQGZS> select undo_sql from flashback_transaction_query
2 where xid = hextoraw('0300280024190000');
Elapsed: 00:00:00.01
Execution Plan
----------------------------------------------------------
Plan hash value: 1747778896
-----------------------------------------------------
| Id | Operation | Name |
-----------------------------------------------------
| 0 | SELECT STATEMENT | |
|* 1 | FIXED TABLE FIXED INDEX| X$KTUQQRY (ind:1) |
-----------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
1 - filter("XID"=HEXTORAW('0300280024190000') )
Note
-----
- rule based optimizer used (consider using cbo)
通过执行相应的UNDO语句可以撤销该事务，通过这些新特性，Oracle提供了一种“回滚”提交事务的手段，极大地方便了用户应对不同情况的数据库恢复。