Oracle事务

Oracle事务的ACID特性

原子性Atomicity：事务中的所有动作要么都发生，要么都不发生。

一致性Consistency：事务将数据库从一种状态转变为下一种一致状态。

隔离性Isolation：一个事务的影响在该事务提交前对其他事务时不可见的。

持久性Durability：事务一旦提交，其影响就是永久性的。

事务控制语句

事务隐式开始

事务必须显示结束（Commit、Rollback）

Commit 会结束事务，并使得已做的修改持久的保存在数据库中

Rollback 会结束事务，并撤销这个事务所做的修改。撤销动作需要读取回滚段中的信息，并把数据恢复到事务开始之前的状态

Savepoint 创建标记点（marked point），一个事务可以有多个标记点

Rollback To<Savepoint> 把事务回滚到指定的标记点，但是不回滚此标记点之前的工作。

Set transaction 允许设置不同的事务属性，如事务的隔离级别以及事务时只读的还是可读可写的。

原子性

语句级原子性

SQL> create table t2(cnt int);

Table created.

SQL> insert into t2 values(0);

1 row created.

SQL> create table t(x int check(x>0));

Table created.

SQL> create or replace trigger t_trigger
  2  before insert or delete on t for each row
  3  begin
  4    if ( inserting ) then
  5        update t2 set cnt=cnt+1;
  6    else
  7        update t2 set cnt=cnt-1;
  8    end if;
  9    dbms_output.put_line('I fired and updated'||sql%rowcount||'rows');
10  end;
11  /

SQL> set serveroutput on
SQL> select * from t2;

CNT
----------
     0

SQL> select * from t;

no rows selected

SQL> insert into t values(1);
I fired and updated1rows

1 row created.

SQL> insert into t values(-1);
I fired and updated1rows
insert into t values(-1)
*
ERROR at line 1:
ORA-02290: check constraint (EODA.SYS_C007120) violated

SQL> select * from t2;

CNT
----------
     1

过程级原子性
Oracle把PL/SQL匿名块也当做是语句

SQL> create or replace procedure p
  2  as
  3  begin
  4     insert into t values(1);
  5     insert into t values(-1);
  6  end;
  7  /

Procedure created.

SQL> delete from t;

0 rows deleted.

SQL> update t2 set cnt=0;

1 row updated.

SQL> commit;

Commit complete.

SQL> select * from t;

no rows selected

SQL> select * from t2;

CNT
----------
     0

SQL> begin
  2  p;
  3  end;
  4  /
begin
*
ERROR at line 1:
ORA-02290: check constraint (EODA.SYS_C007120) violated
ORA-06512: at "EODA.P", line 5
ORA-06512: at line 2

SQL> set serveroutput on
SQL> /
I fired and updated1rows
I fired and updated1rows
begin
*
ERROR at line 1:
ORA-02290: check constraint (EODA.SYS_C007120) violated
ORA-06512: at "EODA.P", line 5
ORA-06512: at line 2

SQL> select * from t;

no rows selected

SQL> select * from t2;

CNT
----------
     0

事务级原子性

DDL与原子性

持久性

COMMIT的WRITE扩展
      COMMIT WRITE WAIT （默认）必须等待redo写到磁盘才返回消息给客户端
      COMMIT WRITE NOWAIT 异步提交，redo条目还没写入磁盘就返回消息给客户端

非分布式PL/SQL代码块中的COMMIT
      在后台以异步的方式执行
      即commit=commit write nowait

完整性约束和事务

Immediate约束  
     整个SQL语句得到处理后立即检查

SQL> create table t(x int unique)；

Table created.

SQL> insert into t values(1);

1 row created.

SQL> insert into t values(2);

1 row created.

SQL> commit;

Commit complete.

SQL> update t set x=x-1;

2 rows updated.

Deferrable约束和级联更新

SQL> create table parent
  2  (pk int primary key)
  3  /

Table created.

SQL> create table child
  2  (fk constraint child_fk_parent
  3      references parent(pk)
  4      deferrable
  5      initially immediate
  6  )
  7  /

Table created.

SQL> insert into parent values(1);

1 row created.

SQL> insert into child values(1);

1 row created.

SQL> update parent set pk=2;
update parent set pk=2
*
ERROR at line 1:
ORA-02292: integrity constraint (EODA.CHILD_FK_PARENT) violated - child record found

由于约束是IMMEDIATE模式，update失败，换为deferred模式
SQL> set constraint child_fk_parent deferred;

Constraint set.

SQL> update parent set pk=2;

1 row updated.

SQL> set constraint child_fk_parent immediate;
set constraint child_fk_parent immediate
*
ERROR at line 1:
ORA-02291: integrity constraint (EODA.CHILD_FK_PARENT) violated - parent key not found

SQL> update child set fk=2;

1 row updated.

SQL> set constraint child_fk_parent immediate;

Constraint set.

SQL> commit;

Commit complete.

不好的事务习惯

在循环中提交
   1.性能影响

SQL> create table t as select * from all_objects;

Table created.

SQL> exec dbms_stats.gather_table_stats(user,'T');

PL/SQL procedure successfully completed.

SQL> variable n number
SQL> exec :n:=dbms_utility.get_cpu_time;

PL/SQL procedure successfully completed.

SQL> update t set object_name=lower(object_name);

17977 rows updated.

SQL> exec dbms_output.put_line((dbms_utility.get_cpu_time-:n)||' cpu hsecs...');
25 cpu hsecs...

PL/SQL procedure successfully completed.

SQL> exec :n:=dbms_utility.get_cpu_time;

PL/SQL procedure successfully completed.

BEGIN
   FOR x IN (SELECT ROWID rid, object_name, ROWNUM r FROM t)
   LOOP
      UPDATE t
         SET object_name = LOWER (x.object_name)
       WHERE ROWID = x.rid;

IF (MOD (x.r, 100) = 0)
      THEN
         COMMIT;
      END IF;
   END LOOP;

COMMIT;
END;

/

PL/SQL procedure successfully completed.

SQL> exec dbms_output.put_line((dbms_utility.get_cpu_time-:n)||' cpu hsecs...');
108 cpu hsecs...

PL/SQL procedure successfully completed.

SQL> exec :n:=dbms_utility.get_cpu_time;

PL/SQL procedure successfully completed.

DECLARE
   TYPE ridArray IS TABLE OF ROWID;

TYPE vcArray IS TABLE OF t.object_name%TYPE;

l_rids    ridArray;
   l_names   vcArray;

CURSOR c
   IS
      SELECT ROWID, object_name FROM t;
BEGIN
   OPEN c;

LOOP
      FETCH c
      BULK COLLECT INTO l_rids, l_names
      LIMIT 100;

FORALL i IN 1 .. l_rids.COUNT
         UPDATE t
            SET object_name = LOWER (l_names (i))
          WHERE ROWID = l_rids (i);

COMMIT;
      EXIT WHEN c%NOTFOUND;
   END LOOP;

CLOSE c;
END;

/

PL/SQL procedure successfully completed.

SQL> exec dbms_output.put_line((dbms_utility.get_cpu_time-:n)||' cpu hsecs');
26 cpu hsecs

PL/SQL procedure successfully completed.

2.Snapshot Too Old
   3.可重启的过程需要复杂的逻辑

使用自动提交

分布式事务

自治事务：允许创建一个事务中的事务，它能独立于其父事务提交或者回滚。