参考文档：
ORA-38754: FLASHBACK DATABASE NOT STARTED - REQUIRED REDO LOG IS NOT AVAILABLE (文档 ID 560686.1)

版本：
Oracle Database - Enterprise Edition - Version 10.1.0.2 and later
Information in this document applies to any platform.
***Checked for relevance on 29-Apr-2013***

问题现象：
When trying  flashback database to restore point failed with:

SQL> flashback database to restore point a2;
flashback database to restore point a2
*
ERROR at line 1:
ORA-38754: FLASHBACK DATABASE not started; required redo log is not available
ORA-38761: redo log sequence 2 in thread 1, incarnation 4 could not be accessed

原因：
The flashback rollback database by using flashback logs, then it recovers to desire point in time
using archived log files.
解决方案：
1: If using SQLplus for Flashback database, then you need to have the required archive logs in the archive destination at the time of flashback.

2: Alternate, use RMAN. Issue the same command from the RMAN will restore required Archived Logfiles from Backup.

RMAN> flashback database to restore point a2;

Starting flashback at 01.APR.2008 15:41:40
allocated channel: ORA_DISK_1
channel ORA_DISK_1: sid=154 devtype=DISK


starting media recovery

channel ORA_DISK_1: starting archive log restore to default destination
channel ORA_DISK_1: restoring archive log
archive log thread=1 sequence=1
channel ORA_DISK_1: restoring archive log
archive log thread=1 sequence=2
channel ORA_DISK_1: restoring archive log
archive log thread=1 sequence=3
channel ORA_DISK_1: restoring archive log
archive log thread=1 sequence=4
channel ORA_DISK_1: reading from backup piece
D:\ORACLE\PRODUCT\10.2.0\FLASH_RECOVERY_AREA\DB0101\BACKUPSET\2008_04_01\O1_MF_ANNNN_TAG20080401T154
011_3Z4GT02X_.BKP
channel ORA_DISK_1: restored backup piece 1
piece
handle=D:\ORACLE\PRODUCT\10.2.0\FLASH_RECOVERY_AREA\DB0101\BACKUPSET\2008_04_01\O1_MF_ANNNN_TAG20080
401T154011_3Z4GT02X_.BKP tag=TAG20080401T
154011
channel ORA_DISK_1: restore complete, elapsed time: 00:00:04
channel ORA_DISK_1: deleting archive log(s)
archive log
filename=D:\ORACLE\PRODUCT\10.2.0\FLASH_RECOVERY_AREA\DB0101\ARCHIVELOG\2008_04_01\O1_MF_1_1_3Z4GWYM
K_.ARC recid=47 stamp=650907711
channel ORA_DISK_1: deleting archive log(s)
archive log
filename=D:\ORACLE\PRODUCT\10.2.0\FLASH_RECOVERY_AREA\DB0101\ARCHIVELOG\2008_04_01\O1_MF_1_2_3Z4GWZM
K_.ARC recid=50 stamp=650907711
channel ORA_DISK_1: deleting archive log(s)
archive log
filename=D:\ORACLE\PRODUCT\10.2.0\FLASH_RECOVERY_AREA\DB0101\ARCHIVELOG\2008_04_01\O1_MF_1_3_3Z4GWYN
J_.ARC recid=49 stamp=650907711
channel ORA_DISK_1: deleting archive log(s)
archive log
filename=D:\ORACLE\PRODUCT\10.2.0\FLASH_RECOVERY_AREA\DB0101\ARCHIVELOG\2008_04_01\O1_MF_1_4_3Z4GWYN
0_.ARC recid=48 stamp=650907711
media recovery complete, elapsed time: 00:00:12
Finished flashback at 01.APR.2008 15:42:05