在oracle 12c多租户环境里，我们有时想通过cdb_data_files来查看种子数据库(pdb$seed)的文件信息，会发现没有反馈结果，但你可能会发现网上有些案例里却能通过cdb_data_files这个视图查看到关于pdb$seed的信息。这是怎么回事呢？
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我在12c几个版本的reference里没有找到关于cdb_data_files的详细说明，不过在 Administrator Guide中找到这句话：
the DBA_PDBS and CDB_DATA_FILES views to show the name and location of each data file for all of the PDBs in a CDB, including the CDB seed.
这说明cdb_data_files确实有pdb$seed的信息。
在MOS中，文档(1940806.1) PDB$SEED Datafiles Not Appear In CDB_DATA_FILES 介绍是参数EXCLUDE_SEED_CDB_VIEW 的设置问题。不过对于这个新参数，oracle 12c各个版本的情况都不相同。
1. 首先在oracle 12.1.0.1版本中，没有参数EXCLUDE_SEED_CDB_VIEW。因此，你会发现在这个版本下，你能从cdb_data_files中直接查看到pdb$seed。
SQL> select * from v$version;
BANNER CON_ID
-------------------------------------------------------------------------------- ----------
Oracle Database 12c Enterprise Edition Release 12.1.0.1.0 - 64bit Production 0
PL/SQL Release 12.1.0.1.0 - Production 0
CORE 12.1.0.1.0 Production 0
TNS for Linux: Version 12.1.0.1.0 - Production 0
NLSRTL Version 12.1.0.1.0 - Production 0
SQL> select file_name from cdb_data_files;
FILE_NAME
--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
/u01/dbdata/cdb1/system01.dbf
/u01/dbdata/cdb1/sysaux01.dbf
/u01/dbdata/cdb1/undotbs01.dbf
/u01/dbdata/cdb1/users01.dbf
/u01/dbdata/cdb1/pdbseed/system01.dbf
/u01/dbdata/cdb1/pdbseed/sysaux01.dbf
/u01/dbdata/cdb1/pdb1/system01.dbf
/u01/dbdata/cdb1/pdb1/sysaux01.dbf
/u01/dbdata/cdb1/pdb1/SAMPLE_SCHEMA_users01.dbf
/u01/dbdata/cdb1/pdb1/example01.dbf
10 rows selected.
SQL> select name,value,description from v$parameter where name like '%seed%';
no rows selected
SQL> SELECT ksppinm, ksppstvl, ksppdesc FROM x$ksppi x, x$ksppcv y WHERE x.indx = y.indx AND ksppinm like '%seed%'
KSPPINM KSPPSTVL KSPPDESC
------------------------------ -------------------- ------------------------------------------------------------
_deferred_seg_in_seed TRUE Enable Deferred Segment Creation in Seed
2. 不过在12.1.0.2版本里，oracle引入了参数EXCLUDE_SEED_CDB_VIEW来控制查看pdb$seed
我们可以看到，默认情况下是查询不到pdb$seed的
SQL> select * from v$version;
BANNER CON_ID
-------------------------------------------------------------------------------- ----------
Oracle Database 12c Enterprise Edition Release 12.1.0.2.0 - 64bit Production 0
PL/SQL Release 12.1.0.2.0 - Production 0
CORE 12.1.0.2.0 Production 0
TNS for Linux: Version 12.1.0.2.0 - Production 0
NLSRTL Version 12.1.0.2.0 - Production 0
SQL> select file_name from cdb_data_files;
FILE_NAME
--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
/u01/dbdata/cdb2/system01.dbf
/u01/dbdata/cdb2/sysaux01.dbf
/u01/dbdata/cdb2/undotbs01.dbf
/u01/dbdata/cdb2/users01.dbf
/u01/dbdata/cdb2/pdb2/system01.dbf
/u01/dbdata/cdb2/pdb2/sysaux01.dbf
/u01/dbdata/cdb2/pdb2/SAMPLE_SCHEMA_users01.dbf
/u01/dbdata/cdb2/pdb2/example01.dbf
8 rows selected.
SQL> SELECT ksppinm, ksppstvl, ksppdesc FROM x$ksppi x, x$ksppcv y WHERE x.indx = y.indx AND ksppinm like '%seed_cdb%';
KSPPINM KSPPSTVL KSPPDESC
------------------------------ -------------------- ------------------------------------------------------------
exclude_seed_cdb_view TRUE exclude PDB$SEED from CDB View Result
参数exclude_seed_cdb_view默认值为true，当设置为false时，就能查看到pdbs$seed了
SQL> alter system set exclude_seed_cdb_view=FALSE;
System altered.
SQL> select file_name from cdb_data_files;
FILE_NAME
--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
/u01/dbdata/cdb2/pdb2/system01.dbf
/u01/dbdata/cdb2/pdb2/sysaux01.dbf
/u01/dbdata/cdb2/pdb2/SAMPLE_SCHEMA_users01.dbf
/u01/dbdata/cdb2/pdb2/example01.dbf
/u01/dbdata/cdb2/system01.dbf
/u01/dbdata/cdb2/sysaux01.dbf
/u01/dbdata/cdb2/undotbs01.dbf
/u01/dbdata/cdb2/users01.dbf
/u01/dbdata/cdb2/pdbseed/system01.dbf
/u01/dbdata/cdb2/pdbseed/sysaux01.dbf
10 rows selected.
3. 而在新版本12.2.0.1中，oracle已经把 EXCLUDE_SEED_CDB_VIEW作为隐含参数了，默认值同样为TURE。
SQL> select * from v$version;
BANNER CON_ID
-------------------------------------------------------------------------------- ----------
Oracle Database 12c Enterprise Edition Release 12.2.0.1.0 - 64bit Production 0
PL/SQL Release 12.2.0.1.0 - Production 0
CORE 12.2.0.1.0 Production 0
TNS for Linux: Version 12.2.0.1.0 - Production 0
NLSRTL Version 12.2.0.1.0 - Production 0
SQL> select file_name from cdb_data_files;
FILE_NAME
---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
/u01/dbdata/cdb3/system01.dbf
/u01/dbdata/cdb3/sysaux01.dbf
/u01/dbdata/cdb3/undotbs01.dbf
/u01/dbdata/cdb3/users01.dbf
/u01/dbdata/cdb3/orclpdb/system01.dbf
/u01/dbdata/cdb3/orclpdb/sysaux01.dbf
/u01/dbdata/cdb3/orclpdb/undotbs01.dbf
/u01/dbdata/cdb3/orclpdb/users01.dbf
8 rows selected.
SQL> SELECT ksppinm, ksppstvl, ksppdesc FROM x$ksppi x, x$ksppcv y WHERE x.indx = y.indx AND ksppinm like '%seed_cdb%';
KSPPINM KSPPSTVL KSPPDESC
------------------------------ -------------------- ------------------------------------------------------------
_exclude_seed_cdb_view TRUE exclude PDB$SEED from CDB View Result
不过好在这个参数可以在instance或session级别修改。
SQL> alter system set "_exclude_seed_cdb_view"=FALSE;
System altered.
SQL> alter session set "_exclude_seed_cdb_view"=FALSE;
Session altered.
SQL> select file_name from cdb_data_files;
FILE_NAME
--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
/u01/dbdata/cdb3/system01.dbf
/u01/dbdata/cdb3/sysaux01.dbf
/u01/dbdata/cdb3/undotbs01.dbf
/u01/dbdata/cdb3/users01.dbf
/u01/dbdata/cdb3/orclpdb/system01.dbf
/u01/dbdata/cdb3/orclpdb/sysaux01.dbf
/u01/dbdata/cdb3/orclpdb/undotbs01.dbf
/u01/dbdata/cdb3/orclpdb/users01.dbf
/u01/dbdata/cdb3/pdbseed/system01.dbf
/u01/dbdata/cdb3/pdbseed/sysaux01.dbf
/u01/dbdata/cdb3/pdbseed/undotbs01.dbf
11 rows selected.
从上面的实验可以看到，oracle越来越重视对pdb$seed的保护。从第一版全部开放，再到12.1.0.2引入普通参数进行查询控制，最后干脆改为隐含参数。
名称栏目：关于在CDB_DATA_FILES视图中如何查看seedpdb的datafile问题
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