ORA-12838: cannot read/modify an object after modifying it in parallel

今天在测试一个parallel实验的时候遇到报错，比较纳闷

ORA-12838: cannot read/modify an object after modifying it in parallel

后来才明白为什么会报错

SQL> insert /*+ append*/ into t1 select * From t2;

71899 rows created.

Elapsed: 00:00:00.04

Execution Plan

----------------------------------------------------------

ERROR:

ORA-12838: cannot read/modify an object after modifying it in parallel

SP2-0612: Error generating AUTOTRACE EXPLAIN report

Statistics

----------------------------------------------------------

0 recursive calls

370 db block gets

347 consistent gets

343 physical reads

2822944 redo size

1116 bytes sent via SQL*Net to client

1292 bytes received via SQL*Net from client

4 SQL*Net roundtrips to/from client

1 sorts (memory)

0 sorts (disk)

71899 rows processed

查看报错的原因：

[oracle@rac1 ~]$ oerr ora 12838

12838, 00000, "cannot read/modify an object after modifying it in parallel"

// *Cause: Within the same transaction, an attempt was made to add read or

// modification statements on a table after it had been modified in parallel

// or with direct load. This is not permitted.

// *Action: Rewrite the transaction, or break it up into two transactions:

// one containing the initial modification and the second containing the

// parallel modification operation.

看了解释以后还是不明白，去网上看了下前人对这个错误的解释

insert /*+ append*/ into t1 select * From t2;

SQL> select * From v$lock where sid=54;

ADDR KADDR SID TY ID1 ID2 LMODE REQUEST CTIME BLOCK

---------------- ---------------- ---------- -- ---------- ---------- ---------- ---------- ---------- ----------

00000000974599B8 0000000097459A10 54 AE 100 0 4 0 15610

0000000097459F68 0000000097459FC0 54 TO 5003 1 3 0 13850

00002B6E0A7FD0D0 00002B6E0A7FD130 54 TM 73443 0 6 0 339 0

00002B6E0A7FD0D0 00002B6E0A7FD130 54 TM 5003 0 3 0 3360

0000000095C9A058 0000000095C9A0D0 54 TX 327705 997 6 0 3390

和普通的insert 对比 lock锁

SQL> insert into t1 select * From t2;

SQL> select * from v$lock where sid=57 ;

ADDR KADDR SID TY ID1 ID2 LMODE REQUEST CTIME BLOCK

---------------- ---------------- ---------- -- ---------- ---------- ---------- ---------- ---------- ----------

0000000097459C28 0000000097459C80 57 AE 100 0 4 0 1230

00002AC39A2CE3E8 00002AC39A2CE448 57 TM 73443 0 3 0 51 0

0000000095D16C98 0000000095D16D10 57 TX 262163 640 6 0 51 0

如上述：

insert /*+ append*/ into t1 select * From t2; 表上有 TM 有个6号锁，排他，必须commit or roll 后才能查看数据。而普通的insert tm上只是3号共享锁

结论：

insert /*+ append*/ into t1 select * From t2;

这样的insert 语句写法是不正确的写法，会产生大量的undo，而且insert后会在表级加上6号锁，在当前session里是不能查询，也不能再一次的插入数据，进行dml操作，否则 会报ORA-12838: cannot read/modify an object after modifying it in parallel错误。

正确的append 写法是这样，

alter table t1 nologging;

SQL> insert /*+ append parallel(2)*/ into t1 select * From t2;

并且在使用append 快速加载数据完成后，需要commit or roll ，才能进行进行对该表进程操作