首先描述一下我自己在学习新知识的时候大概是什么状态，什么思路，因为自认为自己的学习能力还不错，因此也期望这样的学习方法对其他人会有帮助。看这篇文章的时候，你可以同步地想一想如果是你遇到这样的错误，你会怎么处理，怎么发散，怎么研究？
Oracle Database 12c作为最新一代的Oracle数据库产品，已经广为使用，那么，如果学习一个新版本的数据库，应该如何开始呢？
我通常是从New Features Guide文档看起，先通览文档的目录，遇到感兴趣的新功能点，就开始做实验来验证这个新功能。当然，这之前需要先把新版本的数据库安装好、新版本的全部文档下载到本地，这样即使你坐在飞机上也有文档可查。
这次我的计划是实验一下Identity类型的字段，这个字段可以用来作主键，会自动递增，这种类型的字段在SQL Server中早就存在，但是Oracle直到12c才推出这个功能。
通常我不会用sys用户进行任何实验（除非是验证sysdba的新功能），因此总是会先创建一个我自己的dba用户。
在12c中创建这个用户首先就遇到了错误（测试环境启用了多租户架构）。
SQL> CREATE USER kamus IDENTIFIED BY oracle DEFAULT tablespace users; ERROR at line 1: ORA-65096: invalid common USER OR ROLE name复制
对于一个不熟悉的错误，第一件事情不是去Google，而是用oerr实用程序来看看Oracle自己对这个错误是怎么解释的。为什么我喜欢非Windows环境中的Oracle？oerr的存在也是很大一个原因。
[oracle@dbserver-oel ~]$ oerr ora 65096 65096, 00000, "invalid common user or role name" // *Cause: An attempt was made to create a common user or role with a name // that wass not valid for common users or roles. In addition to // the usual rules for user and role names, common user and role // names must start with C## or c## and consist only of ASCII // characters. // *Action: Specify a valid common user or role name.复制
错误信息的解析非常明确地告知“试图创建一个通用用户，必需要用C## 或者c##开头”，这时候心里会有疑问，什么是common user？但是我通常不会先急着去翻文档，而是先把手头的事情做完，也就是先把用户创建上。
SQL> CREATE USER c##kamus IDENTIFIED BY oracle DEFAULT tablespace users; USER created. SQL> GRANT dba TO c##kamus; GRANT succeeded.复制
创建C##KAMUS用户成功之后，再返回去解决心中的疑问，什么是common user？在联机文档的左上角搜索关键字common user，会得到如图5-1所示的结果。
图5-1
通常我会先浏览目录，如果看完觉得心中疑问已经解决，就会返回继续做之前的实验，不会再浏览其他的链接；如果想要查询怎么做，比如说如何创建common user，才会继续去看正文部分。这样的好处是可以保持专注不至于被过多文档分心。
但是由于common user这个概念几乎是崭新的，所以我很有兴趣继续探索一下：跟common user相对的local user该如何创建。继续去看正文当然是个方法，但是这里我选择的是直接去看SQL Language Reference，因为我们知道一定是在Create User语法里面会有不同的定义，进入Create User语法页面，直接搜索common user，就可以看到如下这段话。
CONTAINER Clause：
To create a local user in a pluggable database （PDB）， ensure that the current container is that PDB and specify CONTAINER = CURRENT. To create a common user， ensure that the current container is the root and specify CONTAINER = ALL. The name of the common user must begin with C## or c##. If you omit this clause and the current container is a PDB， then CONTAINER = CURRENT is the default. If you omit this clause and the current container is the root， then CONTAINER = ALL is the default.
也就是说我们一定要先登录进一个PDB，才可以创建本地用户，那么如何知道现在的SQL*Plus是登录进了哪个DB呢？这个疑问其实是一个很简单的联想，既然需要去一个地方，那么一定有方法知道我现在在什么地方，通过简单地查询文档，可以得知以下的方法。现在确实在CDB中。
SQL> SHOW con_name CON_NAME ------------------------------ CDB$ROOT SQL> SELECT SYS_CONTEXT ('USERENV', 'CON_NAME') FROM DUAL; SYS_CONTEXT('USERENV','CON_NAME') ----------------------------------------------- CDB$ROOT复制
dbca建库的时候，有一个新选项是“同时创建PDB”，我勾选过（对于dbca中出现的新选项，如果不是条件不允许，我都会选中进行测试），创建了名字为pdbtest的PDB，那么现在我想尝试登录这个PDB，去创建一个local user。
如何登录PDB？Administrator’s Guide中有专门的一个章节“Part VI Managing a Multitenant Environment”来描述如何管理多租户环境，浏览目录就可以直接找到“Connecting to a PDB with SQL*Plus”这部分，如下所示。
You can use the following techniques to connect to a PDB with the SQL*Plus CONNECT command:
Database connection using easy connect
Database connection using a net service name
那尝试直接使用easy connect来登录PDB。
$ SQLplus sys/oracle@127.0.0.1:15210/pdbtest AS sysdba SQL*Plus: Release 10.2.0.4.0 - Production ON Sat Jul 6 21:44:42 2013 Copyright (c) 1982, 2007, Oracle. ALL Rights Reserved. Connected TO: Oracle DATABASE 12c Enterprise Edition Release 12.1.0.1.0 - 64bit Production WITH the Partitioning, OLAP, Advanced Analytics AND REAL Application Testing options复制
进行如下操作。
SQL> SELECT SYS_CONTEXT ('USERENV', 'CON_NAME') FROM DUAL; SYS_CONTEXT('USERENV','CON_NAME') ------------------------------------------ PDBTEST SQL> SELECT NAME,PDB FROM dba_services; ERROR at line 1: ORA-01219: DATABASE OR pluggable DATABASE NOT OPEN: queries allowed ON fixed TABLES OR views ONLY复制
PDB没有Open？尝试打开。无法使用startup命令。原因是我使用了旧版本的SQL*Plus（如上所示是10.2.0.4.0）连接到12c数据库的PDB中，某些新特性不被支持。
SQL> startup ORA-24543: instance startup OR shutdown NOT allowed IN pluggable DATABASE复制
使用12c自带的SQLPlus登录，就可以使用startup命令将PDB打开，使用SQLPlus管理PDB的详细命令可以参看文档描述。
或者可以使用如下语句打开PDB。
SQL> SHOW USER USER IS "SYS" SQL> startup Pluggable DATABASE opened. SQL> SHOW con_name CON_NAME ------------------------------ PDBTEST SQL> ALTER PLUGGABLE DATABASE OPEN; SQL> SELECT NAME, OPEN_MODE, RESTRICTED, OPEN_TIME FROM V$PDBS; NAME OPEN_MODE RES OPEN_TIME --------- ---------- --- -------------------------- PDBTEST READ WRITE NO 06-JUL-13 09.48.57.260 PM 到此，可以创建local user了。 SQL> CREATE USER kamus IDENTIFIED BY oracle; USER created. SQL> GRANT dba TO kamus; GRANT succeeded.复制
那么在一个PDB中可以看到多少用户呢？可以看到CDB中的用户吗？
这又是一个简单的联想，学习的过程其实是一个发散再收缩的循环。看来不可以，只能看到自己的用户，当然这里有很多common user。可以看到即使是在PDB中，cdb_视图也是可以使用的。
SQL> SELECT CON_ID,COUNT(*) FROM cdb_users GROUP BY con_id; CON_ID COUNT(*) ---------- ---------- 3 38复制
再回到CDB中看一下，会是什么情况？可以看到所有容器数据库中的用户都可以查询到。
SQL> SELECT CON_ID,COUNT(*) FROM cdb_users GROUP BY con_id; CON_ID COUNT(*) ---------- ---------- 1 36 2 35 3 38复制
终于，我可以回到最开始的实验目标上去了，在PDB中创建了T1表，id列为Identity类型。
SQL> CREATE TABLE t1 (id NUMBER GENERATED AS IDENTITY); TABLE created.复制
根据文档描述，Identity类型仍然是通过Sequence来实现的，那么应该是自动创建了一个Sequence，果然如此。在你学习的过程中会多此一步来查询一下Sequence视图吗？
SQL> SELECT SEQUENCE_NAME FROM user_sequences; SEQUENCE_NAME --------------------------------------------------------------- ISEQ$$_91620复制
默认创建的Sequence，CACHE_SIZE是20，开始值是1，这都跟单独创建的Sequence默认值一样。
SQL> SELECT * FROM user_sequences; SEQUENCE_NAME INCREMENT_BY C O CACHE_SIZE LAST_NUMBER PARTITION_COUNT S K MIN_VALUE MAX_VALUE ------------- ------------------------- - - ---------- ----------- --------------- - - ISEQ$$_91620 1 N N 20 1 N N复制
插入一条数据试一下，报错报错还是报错。所以是generated always的identity列，如果只有这一列，就没法插入数据。
SQL> INSERT INTO t1 VALUES(''); ERROR at line 1: ORA-32795: cannot INSERT INTO a generated always IDENTITY COLUMN SQL> INSERT INTO t1 VALUES(ISEQ$$_91620.NEXTVAL); ERROR at line 1: ORA-32795: cannot INSERT INTO a generated always IDENTITY COLUMN SQL> INSERT INTO t1 VALUES(NULL); ERROR at line 1: ORA-32795: cannot INSERT INTO a generated always IDENTITY COLUMN复制
换GENERATED BY DEFAULT ON NULL 类型试一下，Wait，如果删除了表，对应的序列会自动删除吗？理论上应该会，当然还是要测试一下。
SQL> DROP TABLE t1; TABLE dropped.复制
序列还在？
SQL> SELECT * FROM user_sequences; SEQUENCE_NAME INCREMENT_BY C O CACHE_SIZE LAST_NUMBER PARTITION_COUNT S KMIN_VALUE MAX_VALUE ----------------------------------------------------------------- ------------ - - ---------- ----------- --------------- - - ISEQ$$_91620 1 N N 20 1 N N Elapsed: 00:00:00.00复制
再建一张测试表。
SQL> CREATE TABLE t2 (id NUMBER GENERATED BY DEFAULT AS IDENTITY); TABLE created.复制
现在是2个序列了。
SQL> SELECT * FROM user_sequences; SEQUENCE_NAME MIN_VALUE MAX_VALUE INCREMENT_BY C O CACHE_SIZE LAST_NUMBER PARTITION_COUNT S K -------------------------------------- - - ---------- ----------- --------------- - - ISEQ$$_91620 1 1.0000E+28 1 N N 20 1 N N ISEQ$$_91622 0000E+28 1 N N 20 1 N N复制
写完整的Drop语句试一下。
SQL> DROP TABLE t2 cascade CONSTRAINT purge; TABLE dropped.复制
后面创建的序列已经被自动删除了，之前创建的还在。
SQL> SELECT * FROM user_sequences; SEQUENCE_NAME MIN_VALUE MAX_VALUE -------------------------------------------------------------------------- ---------- INCREMENT_BY C O CACHE_SIZE LAST_NUMBER PARTITION_COUNT S K ------------ - - ---------- ----------- --------------- - - ISEQ$$_9162 1.0000E+28 1 N N 20 1 N N复制
两者的不同应该是purge，如果被删除的表还在回收站中，序列是会保留的，因为表还可能从回收站里面再restore回来，需要保证序列仍然有效。那么清空回收站实验一下。
SQL> purge recyclebin; Recyclebin purged.复制
果然，相应的序列也被删除了。
SQL> SELECT * FROM user_sequences; no ROWS selected复制
再回到正题，创建T3表，插入一条数据。
SQL> CREATE TABLE t3 (id NUMBER GENERATED BY DEFAULT ON NULL AS IDENTITY); TABLE created SQL> INSERT INTO t3 VALUES(NULL); 1 ROW created.复制
序列的LAST_NUMBER已经增加为21。
SQL> SELECT * FROM user_sequences; SEQUENCE_NAME MIN_VALUE MAX_VALUE INCREMENT_BY C O CACHE_SIZE LAST_NUMBER PARTITION_COUNT S K ------------ - - ---------- ----------- --------------- - - ISEQ$$_91624 1.0000E+28 1 N N 20 21 N N复制
后台如何操作的？使用10046 trace，再插入几条数据。
SQL> INSERT INTO t3 VALUES(NULL); 1 ROW created. SQL> INSERT INTO t3 VALUES(NULL); 1 ROW created. SQL> SELECT * FROM t3; ID ---------- 1 2 3复制
查看10046 trace的结果。可以看到执行计划中直接调用了SEQUENCE，就跟之前插入记录的时候明确指定SEQ.NEXTVAL一样。其实Oracle的实现方法非常简单，这一列其实就是Number类型，然后将这一列的Default值设置为”KAMUS”.”ISEQ$$_91624″.nextval，仅此而已。
insert into t3 values (null) call count cpu elapsed disk query current rows ------- ------ ------ ------- -------- -------- ------- -------- Parse 1 0.00 0.00 0 0 0 0 Execute 1 0.00 0.00 0 1 3 1 Fetch 0 0.00 0.00 0 0 0 0 ------- ------ ------ -------- -------- -------- ------- ------ total 2 0.00 0.00 0 1 3 1 Misses in library cache during parse: 1 Optimizer mode: ALL_ROWS Parsing user id: 104 Number of plan statistics captured: 1 Rows (1st) Rows (avg) Rows (max) Row Source Operation ---------- ---------- ---------- ------------------------------------------- 0 0 0 LOAD TABLE CONVENTIONAL (cr=1 pr=0 pw=0 time=90 us) 1 1 1 SEQUENCE ISEQ$$_91624 (cr=0 pr=0 pw=0 time=14 us) Elapsed times include waiting on following events: Event waited on Times Max. Wait Total Waited ---------------------------------------- Waited ---------- ------------ SQL*Net message to client 1 0.00 0.00 SQL*Net message from client 1 5.28 5.28 ****************************************************************************复制
使用DBMS_METADATA.GET_DDL获取到的DDL信息，已经符合12c语法的样式了，显示出了Sequence的具体信息。
SQL> SELECT dbms_metadata.GET_DDL('TABLE','T3') FROM dual; DBMS_METADATA.GET_DDL('TABLE','T3') ------------------------------------------------------------------ CREATE TABLE "KAMUS"."T3" ( "ID" NUMBER GENERATED BY DEFAULT ON NULL AS IDENTITY MINVALUE 1 MAXVALUE 99 99999999999999999999999999 INCREMENT BY 1 START WITH 1 CACHE 20 NOORDER NOCYCLE NOT NULL ENABLE, "COMMENTS" VARCHAR2(100) ) SEGMENT CREATION IMMEDIATE PCTFREE 10 PCTUSED 40 INITRANS 1 MAXTRANS 255 NOCOMPRESS LOGGING STORAGE(INITIAL 65536 NEXT 1048576 MINEXTENTS 1 MAXEXTENTS 2147483645 PCTINCREASE 0 FREELISTS 1 FREELIST GROUPS 1 BUFFER_POOL DEFAULT FLASH_CACHE DEFAULT CELL_FLASH_CACHE DEFAULT) TABLESPACE "USERS"复制
系统自动产生的序列无法手工修改属性。
SQL> ALTER SEQUENCE "ISEQ$$_91624" INCREMENT BY 10; ERROR at line 1: ORA-32793: cannot ALTER a system-generated SEQUENCE SQL> host oerr ora 32793 32793,0000, "cannot alter a system-generated sequence" // *Cause: An attempt was made TO ALTER a system-generated SEQUENCE. // *Action: A system-generated SEQUENCE, such AS one created FOR an // IDENTITY COLUMN, cannot be altered.复制
系统自动产生的序列也不允许删除。
SQL> DROP SEQUENCE "ISEQ$$_91624"; ERROR at line 1: ORA-32794: cannot DROP a system-generated SEQUENCE SQL> host oerr ora 32794 32794,0000, "cannot drop a system-generated sequence" // *Cause: An attempt was made TO DROP a system-generated SEQUENCE. // *Action: A system-generated SEQUENCE, such AS one created FOR an // IDENTITY COLUMN, cannot be dropped.复制
在11gR2中，错误信息编号在ORA-32790和ORA-32800之间是空白，而12c使用了这其间的8个错误号作为新特性的报错。
ORA-32791: prebuilt table managed column cannot have a default on null Cause: An attempt was made to create a materialized view on a prebuilt table that has a managed column with a default on null expression. Action: Either remove the default on null property， or do not include the column in the materialized view definition. ORA-32792: prebuilt table managed column cannot be an identity column Cause: An attempt was made to create a materialized view on a prebuilt table that has a managed column that is an identity column. Action: Either remove the identity property， or do not include the column in the materialized view definition. ORA-32793: cannot alter a system-generated sequence Cause: An attempt was made to alter a system-generated sequence. Action: A system-generated sequence， such as one created for an identity column， cannot be altered. ORA-32794: cannot drop a system-generated sequence Cause: An attempt was made to drop a system-generated sequence. Action: A system-generated sequence， such as one created for an identity column， cannot be dropped. ORA-32795: cannot insert into a generated always identity column Cause: An attempt was made to insert a value into an identity column created with GENERATED ALWAYS keywords. Action: A generated always identity column cannot be directly inserted. Instead， the associated sequence generator must provide the value. ORA-32796: cannot update a generated always identity column Cause: An attempt was made to update an identity column created with GENERATED ALWAYS keywords. Action: A generated always identity column cannot be directly updated. ORA-32797: identity column sequence mismatch in ALTER TABLE EXCHANGE PARTITION Cause: The two tables specified in the EXCHANGE have identity columns with sequences that are neither both increasing nor decreasing. Action: Ensure that the identity columns have sequences with INCREMENT BY having the same sign. ORA-32798: cannot use ANSI RIGHT or FULL outer join with a left correlation Cause: An attempt was made to use a lateral view with a left correlation to the first operand of an ANSI RIGHT or FULL outer join. Action: Rewrite the query without the left correlation.复制
到此为止可以休息一下了，从ORA-65096开始大概花费了1个多小时的时间，我学习到了：
（1）什么是common user，什么是local user？
（2）如何查询现在的环境是CDB还是某个PDB？
（3）如何登录PDB？
（4）如何启动PDB？
（5）PDB和CDB中视图看到的内容有怎样的不同？
（6）如何创建Identity类型的列？
（7）删除表以后，对应的Sequence如何处理？
（8）Oracle后台对于Identity列是如何处理的？
你是不是也是这样学习的呢？
