Direct Path Insert与APPEND, PARALLEL的梳理与小结

Oracle里面的写入数据方式分为2种，一个是传统方式写入(conventional insert)，另外一个是直接路径写入(direct path insert)。

            传统方式写入: Oracle会重用表里面空闲空间，并且写入会先写入到buffer cache。

            直接路径写入：Oracle不重用表里面的空闲空间，直接写入到新分配的块，并且数据直接写入到data file，不写入到buffer cache。效率更高。如果表的并行度不为1，那么直接路径写入是默认的写入行为（需要开启会话并行dml）。直接路径写入为元数据（例如空间扩展引起的数据字典数据变化）变更产生REDO, UNDO。而数据变更，redo，undo生成情况如下：

Ø  不为数据写入生成undo

Ø  如果数据库为非归档或者没有开启force logging，那么直接路径写入时，不为数据写入产生redo，这时与表是否设置nologging属性无关。

Ø  如果数据库为归档，但是没有开启force logging，logging的表会产生redo，nologging的表不会产生redo。

Ø  如果数据库为归档并且开启了force logging，数据写入会产生redo，无论是否设置logging，nologging。

            

            直接路径写入的一些限制：

Ø  在一个事务里面，可以执行多次直接路径写入，一旦该事物并行修改了表，分区或者索引，当前会话将无法再次方位该对象。例如会话A在表T上进行直接路径写入后，会话A无法再次访问表T，但是会话B可以查询表T，此时如果会话A执行一个传统路径写入，会被阻塞，等待事件为：enq: TM – contention。

Ø  目标表不能是cluster，不能含有object类型字段，不能含有外键约束，触发器。

Ø 对于目标表是IOT表，也有一些限制。

Ø  分布式写入无法进行直接路径写入。

 

APPEND,PARALLE和直接路径写入关系：

                Append默认也是采用直接路径写入，不要求session enable parallel。

                Parallel 要求会话级别enable parallel，否则将无法进行直接路径写入。

测试如下：

 SQL> select log_mode, force_logging from v$database;

 LOG_MODE         FORCE_
 ------------------------ ------
 ARCHIVELOG       NO

 SQL> select * from v$version;

 BANNER
 -------------------------------------------------------------------------------------------------------------------
 Oracle Database 11g Enterprise Edition Release 11.2.0.3.0 - 64bit Production
 PL/SQL Release 11.2.0.3.0 - Production
 CORE    11.2.0.3.0  Production
 TNS for Linux: Version 11.2.0.3.0 - Production
 NLSRTL Version 11.2.0.3.0 - Production

 ===scott测试用户，构造测试数据源和数据表
 SQL> conn scott/tiger;
 Connected.
 SQL> create table t_data_pool as select * from dba_objects;

 Table created.

 SQL> create table t_direct_test as select * from t_data_pool where 1=2;

 Table created.

 SQL>

1、测试REDO生成的量，以及直接路径阻塞传统写入的场景

 ==场景1，Redo的量
 SQL> set autot on;
 SQL> insert into t_direct_test select * from t_data_pool;

 75696 rows created.


 Execution Plan
 ----------------------------------------------------------
 Plan hash value: 2724272089

 ------------------------------------------------------------------------------------------
 | Id  | Operation        | Name      | Rows  | Bytes | Cost (%CPU)| Time     |
 ------------------------------------------------------------------------------------------
 |   0 | INSERT STATEMENT     |       | 76299 |    15M|   216   (1)| 00:00:03 |
 |   1 |  LOAD TABLE CONVENTIONAL | T_DIRECT_TEST |   |   |        |      |
 |   2 |   TABLE ACCESS FULL  | T_DATA_POOL   | 76299 |    15M|   216   (1)| 00:00:03 |
 ------------------------------------------------------------------------------------------

 Note
 -----
    - dynamic sampling used for this statement (level=2)


 Statistics
 ----------------------------------------------------------
     266  recursive calls
       10388  db block gets
        3228  consistent gets
     784  physical reads
     8845208  redo size           ==> redo 量
     837  bytes sent via SQL*Net to client
     807  bytes received via SQL*Net from client
       3  SQL*Net roundtrips to/from client
       2  sorts (memory)
       0  sorts (disk)
       75696  rows processed

 SQL> commit;

 Commit complete.

 ==使用append， logging时，redo量测试如下：
 SQL> insert /*+append*/ into t_direct_test select * from t_data_pool;

 75696 rows created.


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


 SP2-0612: Error generating AUTOTRACE EXPLAIN report

 Statistics
 ----------------------------------------------------------
      94  recursive calls
        1397  db block gets
        1204  consistent gets
       0  physical reads
     8890936  redo size                       ==>redo 量
     823  bytes sent via SQL*Net to client
     819  bytes received via SQL*Net from client
       3  SQL*Net roundtrips to/from client
       1  sorts (memory)
       0  sorts (disk)
       75696  rows processed

 SQL> commit;

 Commit complete


 ==nologging append量，明显很少了
 SQL> alter table t_direct_test nologging;

 Table altered.

 SQL> insert /*+append*/ into t_direct_test select * from t_data_pool;

 75696 rows created.


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


 SP2-0612: Error generating AUTOTRACE EXPLAIN report

 Statistics
 ----------------------------------------------------------
     139  recursive calls
        1361  db block gets
        1229  consistent gets
       0  physical reads
      20904  redo size                              ==>Redo 量
     824  bytes sent via SQL*Net to client
     819  bytes received via SQL*Net from client
       3  SQL*Net roundtrips to/from client
       7  sorts (memory)
       0  sorts (disk)
       75696  rows processed

 SQL> select count(1) from t_direct_test;
 select count(1) from t_direct_test
                      *
 ERROR at line 1:
 ORA-12838: cannot read/modify an object after modifying it in parallel
 ==此时不提交，当前事务是无法访问到该对象的，其他session可以。

新开会话B，

 oracle@ora11gr2 ~]$sqlplus scott/tiger
 SQL*Plus: Release 11.2.0.3.0 Production on Wed Jun 11 12:30:31 2014

 Copyright (c) 1982, 2011, Oracle.  All rights reserved.


 Connected to:
 Oracle Database 11g Enterprise Edition Release 11.2.0.3.0 - 64bit Production
 With the Partitioning, OLAP, Data Mining and Real Application Testing options

 SQL> select count(1) from t_direct_test;

   COUNT(1)
 ----------
     151392

 SQL> insert into t_direct_test select * from t_data_pool;  ==会被阻塞

新开会话C，查看锁的情况：

 当前会话申请0号TM锁被直接路径写入会话阻塞，直接路径写入会话持有Exclusive(6) TM锁。
 col resource format a20
 col sid format 9999
 col request for a15
 Select Type || '-' || Id1 || '-' || Id2 "resource",
        Sid,
        Decode(Lmode,
               0,
               'None',
               1,
               'Null',
               2,
               'Row share',
               3,
               'Row Exclusive',
               4,
               'Share',
               5,
               'Share Row Exclusive',
               6,
               'Exlusive') Lock_Type,
        Decode(Request,
               0,
               'None',
               1,
               'Null',
               2,
               'Row share',
               3,
               'Row Exclusive',
               4,
               'Share',
               5,
               'Share Row Exclusive',
               6,
               'Exlusive') Request,
        Ctime,
        Block
   From V$lock
  Where Type In ('TX', 'TM')
  37   Order By "resource", Ctime Desc;

 resource           SID LOCK_TYPE                  REQUEST          CTIME      BLOCK
 -------------------- ----- -------------------------------------- --------------- ---------- ----------
 TM-5124-0       44 Row Exclusive              None           905          0
 TM-78585-0      29 Row Exclusive              None          2912          0
 TM-78587-0      44 Exlusive               None           906          1       ====> TM上持有6号锁
 TM-78587-0      58 None                   Row Exclusive      380          0       ====> TM上被阻塞
 TX-262166-8704      44 Exlusive               None           906          0
 TX-458782-8781      29 Exlusive               None          2912          0

 6 rows selected.

 SQL>

 Sid为44的正在执行直接路径插入，当时还没有提交，而58号 会话在执行一个传统路径写入

提交44号会话，58号 会话完成

2、测试append，parallel的默认插入方式

上面的测试已经表名，在表默认的并行度为1的情况下，直接append是会选择直接路径写入方式。下面现将表t_direct_test的并行度设置为4.

 SQL> conn scott/tiger
 Connected.
 SQL> col owner for a10
 SQL> col degree for 999
 SQL> col table_name for a20
 SQL> select owner, table_name, Degree from dba_tables Where table_name = Upper('t_direct_test');

 OWNER      TABLE_NAME       DEGREE
 ---------- -------------------- --------------------------------------------------------------------------------
 SCOTT      T_DIRECT_TEST         1

 SQL> alter table t_direct_test parallel 4;

 Table altered.

 SQL> select owner, table_name, Degree from dba_tables Where table_name = Upper('t_direct_test');

 OWNER      TABLE_NAME       DEGREE
 ---------- -------------------- --------------------------------------------------------------------------------
 SCOTT      T_DIRECT_TEST         4

 SQL> insert into t_direct_test select * from t_data_pool;

 75696 rows created.

 SQL> select count(1) from t_direct_test;            ==>没有报错，即表明没有用到直接路径写入

   COUNT(1)
 ----------
 378480

 SQL> commit;

 Commit complete.

 SQL> alter session enable parallel dml;

 Session altered.

 SQL> insert into t_direct_test select * from t_data_pool;

 75696 rows created.

 SQL> select count(1) from t_direct_test;
 ==报错，说明在会话开启允许并行dml的情况下，会直接使用直接路径写入方式。
 select count(1) from t_direct_test
 *
 ERROR at line 1:
 ORA-12838: cannot read/modify an object after modifying it in parallel

 SQL>