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。