对于19c在pdb情况下三种常见故障进行了模拟测试:
模拟19c数据库redo异常恢复
模拟19c数据库pdb undo异常恢复
模拟19c数据库root pdb undo异常恢复
模拟oracle 19c cdb模式下root pdb中undo丢失故障恢复
会话1,pdb中插入大量数据,未提交

SQL> alter session set container=pdb; 

Session altered.

SQL> alter database open;

Database altered.

SQL> create user xff identified by oracle default tablespace users;
grant dba to xff;
conn xff/oracle@127.0.0.1/pdb
create table t_xifenfei as select * from dba_objects;
insert into t_xifenfei select * from t_xifenfei;
insert into t_xifenfei select * from t_xifenfei;
insert into t_xifenfei select * from t_xifenfei;
insert into t_xifenfei select * from t_xifenfei;
insert into t_xifenfei select * from t_xifenfei;

User created.

SQL> 
Grant succeeded.

SQL> Connected.
SQL> 

Table created.

SQL> 
72351 rows created.

SQL> 
144702 rows created.

SQL> 
289404 rows created.

SQL> 
578808 rows created.

SQL> 

1157616 rows created.

SQL> SQL> SQL> 

会话2中root pdb模拟事务

[oracle@localhost ~]$ ss

SQL*Plus: Release 19.0.0.0.0 - Production on Mon Nov 16 16:56:01 2020
Version 19.5.0.0.0

Copyright (c) 1982, 2019, Oracle.  All rights reserved.


Connected to:
Oracle Database 19c Enterprise Edition Release 19.0.0.0.0 - Production
Version 19.5.0.0.0

SQL> 
SQL> 
SQL> conn system/oracle
Connected.
SQL> create table t_xifenfei tablespace users as select * from dba_objects;
insert into t_xifenfei select * from t_xifenfei;
insert into t_xifenfei select * from t_xifenfei;
insert into t_xifenfei select * from t_xifenfei;
insert into t_xifenfei select * from t_xifenfei;
insert into t_xifenfei select * from t_xifenfei;

Table created.

SQL> 
72380 rows created.

SQL> 
144760 rows created.

SQL> 
289520 rows created.

SQL> 
579040 rows created.

SQL> 

1158080 rows created.

SQL> SQL> 

会话3 abort库并删除root pdb中undo文件

[oracle@localhost ~]$ ss

SQL*Plus: Release 19.0.0.0.0 - Production on Mon Nov 16 16:56:55 2020
Version 19.5.0.0.0

Copyright (c) 1982, 2019, Oracle.  All rights reserved.


Connected to:
Oracle Database 19c Enterprise Edition Release 19.0.0.0.0 - Production
Version 19.5.0.0.0

SQL> shutdown abort;
ORACLE instance shut down.
SQL> 

[oracle@localhost oradata]$ cd ORA19C
[oracle@localhost ORA19C]$ ls
control01.ctl  control02.ctl  pdb  pdbseed  redo01.log  redo02.log  redo03.log  sysaux01.dbf
system01.dbf  temp01.dbf  undotbs01.dbf  users01.dbf
[oracle@localhost ORA19C]$ rm -rf undotbs01.dbf 

启动数据库报ORA-01157 ORA-01110错误

SQL> alter database datafile 4 offline drop;

Database altered.

SQL> alter database open;

Database altered.

SQL> show pdbs;

    CON_ID CON_NAME                       OPEN MODE  RESTRICTED
---------- ------------------------------ ---------- ----------
         2 PDB$SEED                       READ ONLY  NO
         3 PDB                            MOUNTED
SQL> alter session set container=pdb;

Session altered.

SQL> alter database open;

Database altered.

SQL> conn / as sysdba
Connected.
SQL> show pdbs;

    CON_ID CON_NAME                       OPEN MODE  RESTRICTED
---------- ------------------------------ ---------- ----------
         2 PDB$SEED                       READ ONLY  NO
         3 PDB                            READ WRITE NO
 
SQL> SQL> select tablespace_name,segment_name,status from dba_rollback_segs;

TABLESPACE_NAME                SEGMENT_NAME                   STATUS
------------------------------ ------------------------------ --------------------------------
SYSTEM                         SYSTEM                         ONLINE
UNDOTBS1                       _SYSSMU1_1261223759$           NEEDS RECOVERY
UNDOTBS1                       _SYSSMU2_27624015$             NEEDS RECOVERY
UNDOTBS1                       _SYSSMU3_2421748942$           NEEDS RECOVERY
UNDOTBS1                       _SYSSMU4_625702278$            NEEDS RECOVERY
UNDOTBS1                       _SYSSMU5_2101348960$           NEEDS RECOVERY
UNDOTBS1                       _SYSSMU6_813816332$            NEEDS RECOVERY
UNDOTBS1                       _SYSSMU7_2329891355$           NEEDS RECOVERY
UNDOTBS1                       _SYSSMU8_399776867$            NEEDS RECOVERY
UNDOTBS1                       _SYSSMU9_1692468413$           NEEDS RECOVERY
UNDOTBS1                       _SYSSMU10_930580995$           NEEDS RECOVERY

本次测试比较幸运,虽然undo段状态为NEEDS RECOVERY,但是数据库直接open成功.实际生产情况,可能比这个要复杂很多