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。