Physical Database Limits

在pre10g的很长时间内，Oracle仅提供exp/imp导入导出工具，虽然这2个实用程序十分有效(现在也是如此)，但因为它们受限于client/server模式工具自身的限制，以普通用户程序的身份来运转数以TB计的数据，其才不堪大用！DataPump是10g以后主推的数据抽取/导入工具，不同于exp/imp工具，DataPump是一个服务器端的实用程序，因为运行在服务器上故而DataPump进程可以直接访问数据文件与SGA(无需借shadow进程之手)，与exp/imp工具相比使用DataPump可以获得显著的性能改善。DataPump可以通过直接路径或外部表路径这两种方法导出数据；其中直接路径避开了数据库高速缓存。当使用直接路径模式抽取数据时，DataPump从磁盘直接读取数据文件，抽取和格式化文件内容，最后将内容写出到转储文件。该种模式和SGA交互等待少，其导入导出速度直接取决于数据库所在磁盘速度和cpu;因此，直接路径极为快速。

外部表路径模式将使用到数据库的高速缓存buffer cache，通过外部表路径方法导出数据时，DataPump使用普通的SELECT操作将数据块从数据文件中读入buffer cache，为了写出转储文件，数据会在缓存中被格式化。通过外部表路径导入数据时，DataPump根据转储文件的内容构造标准的插入语句，并且通过将数据块从数据文件读至缓存来执行这些语句，插入操作按照标准的样式在缓存中完成；如同任何普通DML操作一样，外部表路径也会同时产生撤销和重做。

DataPump自身会根据对象的复杂性作出使用直接路径还是外部表路径的选择；对于较复杂的对象(后文将列出)而言，为了分解复杂性而必须同SGA进行交互，此情况下Data Pump只能采用外部表模式。我们还可以通过使用access_method参数来控制其行为，当然这仅在我们确认Data Pump作出了错误选择时才有必要。

我们有必要深入了解一下外部表路径究竟是如何工作的:

SQL> select * from v$version;
BANNER
----------------------------------------------------------------
Oracle Database 10g Enterprise Edition Release 10.2.0.4.0 - 64bi
PL/SQL Release 10.2.0.4.0 - Production
CORE    10.2.0.4.0      Production
TNS for Linux: Version 10.2.0.4.0 - Production
NLSRTL Version 10.2.0.4.0 - Production

SQL> exec dbms_workload_repository.create_snapshot;
PL/SQL procedure successfully completed.

/*  执行expdp导出任务，TCACHE表大小为465M，这里显示指定了外部表路径 */

[maclean@rh2 ~]$ expdp maclean/maclean directory=dumps tables=TCACHE dumpfile=tcache1.dmp access_method=external_table
Export: Release 10.2.0.4.0 - 64bit Production on Tuesday, 24 August, 2010 20:31:48
Copyright (c) 2003, 2007, Oracle.  All rights reserved.
Connected to: Oracle Database 10g Enterprise Edition Release 10.2.0.4.0 - 64bit Production
With the Partitioning, OLAP, Data Mining and Real Application Testing options
Starting "MACLEAN"."SYS_EXPORT_TABLE_07":  maclean/******** directory=dumps tables=TCACHE dumpfile=tcache1.dmp access_method=external_table
Estimate in progress using BLOCKS method...
Processing object type TABLE_EXPORT/TABLE/TABLE_DATA
Total estimation using BLOCKS method: 544 MB
Processing object type TABLE_EXPORT/TABLE/TABLE
Processing object type TABLE_EXPORT/TABLE/STATISTICS/TABLE_STATISTICS
. . exported "MACLEAN"."TCACHE"                          465.6 MB 4999999 rows
Master table "MACLEAN"."SYS_EXPORT_TABLE_07" successfully loaded/unloaded
******************************************************************************
Dump file set for MACLEAN.SYS_EXPORT_TABLE_07 is:
  /s01/dump/tcache1.dmp
Job "MACLEAN"."SYS_EXPORT_TABLE_07" successfully completed at 20:32:18

/*  这次我们指定了并行度为4  */

[maclean@rh2 ~]$ expdp maclean/maclean directory=dumps tables=TCACHE dumpfile=tcache2.dmp,tcache3.dmp,tcache4.dmp,tcache5.dmp access_method=external_table parallel=4
Export: Release 10.2.0.4.0 - 64bit Production on Tuesday, 24 August, 2010 20:32:49
Copyright (c) 2003, 2007, Oracle.  All rights reserved.
Connected to: Oracle Database 10g Enterprise Edition Release 10.2.0.4.0 - 64bit Production
With the Partitioning, OLAP, Data Mining and Real Application Testing options
Starting "MACLEAN"."SYS_EXPORT_TABLE_07":  maclean/******** directory=dumps tables=TCACHE dumpfile=tcache2.dmp,tcache3.dmp,tcache4.dmp,tcache5.dmp access_method=external_table parallel=4
Estimate in progress using BLOCKS method...
Processing object type TABLE_EXPORT/TABLE/TABLE_DATA
Total estimation using BLOCKS method: 544 MB
Processing object type TABLE_EXPORT/TABLE/TABLE
Processing object type TABLE_EXPORT/TABLE/STATISTICS/TABLE_STATISTICS
. . exported "MACLEAN"."TCACHE"                          465.6 MB 4999999 rows
Master table "MACLEAN"."SYS_EXPORT_TABLE_07" successfully loaded/unloaded
******************************************************************************
Dump file set for MACLEAN.SYS_EXPORT_TABLE_07 is:
  /s01/dump/tcache2.dmp
  /s01/dump/tcache3.dmp
  /s01/dump/tcache4.dmp
Job "MACLEAN"."SYS_EXPORT_TABLE_07" successfully completed at 20:33:04

SQL> exec dbms_workload_repository.create_snapshot;
PL/SQL procedure successfully completed.

SQL> @?/rdbms/admin/awrrpt

接着我们分析该时段内的awr报告!

从AWR报告中，我们不难找到以下语句:
SQL ID 3qwsywpvtdyjp:
CREATE TABLE "ET$088200010001" ( "OWNER", "OBJECT_NAME", "SUBOBJECT_NAME", "OBJECT_ID", "DATA_OBJECT_ID", "OBJECT_TYPE", "CREATED", "LAST_DDL_TIME", "TIMESTAMP", "STATUS", "TEMPORARY", "GENERATED", "SECONDARY" ) ORGANIZATION EXTERNAL ( TYPE ORACLE_DATAPUMP DEFAULT DIRECTORY DUMPS ACCESS PARAMETERS (DEBUG =0 DATAPUMP INTERNAL TABLE "MACLEAN"."TCACHE" JOB ( "MACLEAN", "SYS_EXPORT_TABLE_07", 1) WORKERID 1 PARALLEL 1 VERSION COMPATIBLE ENCRYPTPASSWORDISNULL ) LOCATION ('bogus.dat') ) PARALLEL 1 REJECT LIMIT UNLIMITED AS SELECT /*+ PARALLEL(KU$, 1) */ "OWNER", "OBJECT_NAME", "SUBOBJECT_NAME", "OBJECT_ID", "DATA_OBJECT_ID", "OBJECT_TYPE", "CREATED", "LAST_DDL_TIME", "TIMESTAMP", "STATUS", "TEMPORARY", "GENERATED", "SECONDARY" FROM RELATIONAL("MACLEAN"."TCACHE" ) KU$

SQL ID 1rxax27p7anr3:
CREATE TABLE "ET$088000020001" ( "OWNER", "OBJECT_NAME", "SUBOBJECT_NAME", "OBJECT_ID", "DATA_OBJECT_ID", "OBJECT_TYPE", "CREATED", "LAST_DDL_TIME", "TIMESTAMP", "STATUS", "TEMPORARY", "GENERATED", "SECONDARY" ) ORGANIZATION EXTERNAL ( TYPE ORACLE_DATAPUMP DEFAULT DIRECTORY DUMPS ACCESS PARAMETERS (DEBUG =0 DATAPUMP INTERNAL TABLE "MACLEAN"."TCACHE" JOB ( "MACLEAN", "SYS_EXPORT_TABLE_07", 1) WORKERID 2 PARALLEL 2 VERSION COMPATIBLE ENCRYPTPASSWORDISNULL ) LOCATION ('bogus.dat') ) PARALLEL 2 REJECT LIMIT UNLIMITED AS SELECT /*+ PARALLEL(KU$, 2) */ "OWNER", "OBJECT_NAME", "SUBOBJECT_NAME", "OBJECT_ID", "DATA_OBJECT_ID", "OBJECT_TYPE", "CREATED", "LAST_DDL_TIME", "TIMESTAMP", "STATUS", "TEMPORARY", "GENERATED", "SECONDARY" FROM RELATIONAL("MACLEAN"."TCACHE" ) KU$

以上就是Expdp在导出数据时使用的创建组织类型为ORACLE_DATAPUMP的外部表的SQL语句；后者语句中指定的并行度为2，而非我所指定的4，因该是Oracle考虑到所需导出表的具体大小，设置过高的并行度反而会适得其反，所以对设定值做了修正。

启用了并行的SQL 1rxax27p7anr3执行时间为22.63s，反而要比不使用并行慢；可见在TCACHE表这个数量级，完全没有必要使用并行导出。

接着我们来探究一下外部表路径的导入:

SQL> truncate table tcache;
Table truncated.
SQL> set pagesize 1400;
SQL> set linesize 140;

/* 执行外部表路径的数据导入，并行度1  */

[maclean@rh2 admin]$ impdp maclean/maclean directory=dumps tables=TCACHE content=data_only  dumpfile=tcache1.dmp access_method=external_table

/* 在以上SQL执行期间查询V$SQL动态视图的相关语句*/

select sql_text,
       sql_id,
       cpu_time,
       elapsed_time,
       disk_reads,
       buffer_gets,
       rows_processed
  from v$sql
 where sql_text like '%TCACHE%'
   and sql_text not like '%like%';

SQL_TEXT
--------------------------------------------------------------------------------------------------------------------------------------------
SQL_ID          CPU_TIME ELAPSED_TIME DISK_READS BUFFER_GETS ROWS_PROCESSED
------------- ---------- ------------ ---------- ----------- --------------
INSERT /*+ SYS_DL_CURSOR */ INTO RELATIONAL("MACLEAN"."TCACHE") ("OWNER","OBJECT_NAME","SUBOBJECT_NAME","OBJECT_ID","DATA_OBJECT_ID","OBJECT
_TYPE","CREATED","LAST_DDL_TIME","TIMESTAMP","STATUS","TEMPORARY","GENERATED","SECONDARY") VALUES (NULL,NULL,NULL,NULL,NULL,NULL,NULL,NULL,N
ULL,NULL,NULL,NULL,NULL)
6tn47a220d34q        408          408          0           0              0

INSERT /*+APPEND+*/  /*+PARALLEL("TCACHE",1)+*/ INTO RELATIONAL("MACLEAN"."TCACHE" ) ("OWNER", "OBJECT_NAME", "SUBOBJECT_NAME", "OBJECT_ID",
 "DATA_OBJECT_ID", "OBJECT_TYPE", "CREATED", "LAST_DDL_TIME", "TIMESTAMP", "STATUS", "TEMPORARY", "GENERATED", "SECONDARY")     SELECT "OWNE
R", "OBJECT_NAME", "SUBOBJECT_NAME", "OBJECT_ID", "DATA_OBJECT_ID", "OBJECT_TYPE", "CREATED", "LAST_DDL_TIME", "TIMESTAMP", "STATUS", "TEMPO
RARY", "GENERATED", "SECONDARY"     FROM "ET$087B00060001" KU$
66q4w8c4ak0wf      31576        31576          0         608              0

/* 当导入job完成后再次查询 */
select sql_text,
       sql_id,
       cpu_time,
       elapsed_time,
       disk_reads,
       buffer_gets,
       rows_processed
  from v$sql
 where sql_text like '%TCACHE%'
   and sql_text not like '%like%';

SQL_TEXT
--------------------------------------------------------------------------------------------------------------------------------------------
SQL_ID          CPU_TIME ELAPSED_TIME DISK_READS BUFFER_GETS ROWS_PROCESSED
------------- ---------- ------------ ---------- ----------- --------------
INSERT /*+ SYS_DL_CURSOR */ INTO RELATIONAL("MACLEAN"."TCACHE") ("OWNER","OBJECT_NAME","SUBOBJECT_NAME","OBJECT_ID","DATA_OBJECT_ID","OBJECT
_TYPE","CREATED","LAST_DDL_TIME","TIMESTAMP","STATUS","TEMPORARY","GENERATED","SECONDARY") VALUES (NULL,NULL,NULL,NULL,NULL,NULL,NULL,NULL,N
ULL,NULL,NULL,NULL,NULL)
6tn47a220d34q        408          408          0           0              0

/* 执行完成后一句SQL立马消失了！？ Oracle好像很不情愿我们看到外部表路径使用的INSERT语句 :) */

INSERT /*+APPEND+*/ /*+PARALLEL("TCACHE",1)+*/
INTO RELATIONAL
  ("MACLEAN"."TCACHE")
  ("OWNER",
   "OBJECT_NAME",
   "SUBOBJECT_NAME",
   "OBJECT_ID",
   "DATA_OBJECT_ID",
   "OBJECT_TYPE",
   "CREATED",
   "LAST_DDL_TIME",
   "TIMESTAMP",
   "STATUS",
   "TEMPORARY",
   "GENERATED",
   "SECONDARY")
  SELECT "OWNER",
         "OBJECT_NAME",
         "SUBOBJECT_NAME",
         "OBJECT_ID",
         "DATA_OBJECT_ID",
         "OBJECT_TYPE",
         "CREATED",
         "LAST_DDL_TIME",
         "TIMESTAMP",
         "STATUS",
         "TEMPORARY",
         "GENERATED",
         "SECONDARY"
    FROM "ET$087F00230001" KU$

/* 这里的KU$代表的就是以ORACLE_DATAPUMP转储文件形式存放在数据库外的外部表 */

/* 可以看到这里尝试使用了APPEND插入模式，以减少undo和redo的产生*/

SQL> truncate table tcache;
Table truncated.

/* 若启用并行导入呢？ */

[maclean@rh2 admin]$ impdp maclean/maclean directory=dumps tables=TCACHE content=data_only  dumpfile=tcache2.dmp,tcache3.dmp,tcache4.dmp access_method=external_table parallel=3
........

select sql_text,
       sql_id,
       cpu_time,
       elapsed_time,
       disk_reads,
       buffer_gets,
       rows_processed
  from v$sql
 where sql_text like '%TCACHE%'
   and sql_text not like '%like%';
SQL_TEXT
--------------------------------------------------------------------------------------------------------------------------------------------
SQL_ID          CPU_TIME ELAPSED_TIME DISK_READS BUFFER_GETS ROWS_PROCESSED
------------- ---------- ------------ ---------- ----------- --------------
INSERT /*+APPEND+*/  /*+PARALLEL("TCACHE",1)+*/ INTO RELATIONAL("MACLEAN"."TCACHE" ) ("OWNER", "OBJECT_NAME", "SUBOBJECT_NAME", "OBJECT_ID",
 "DATA_OBJECT_ID", "OBJECT_TYPE", "CREATED", "LAST_DDL_TIME", "TIMESTAMP", "STATUS", "TEMPORARY", "GENERATED", "SECONDARY")     SELECT "OWNE
R", "OBJECT_NAME", "SUBOBJECT_NAME", "OBJECT_ID", "DATA_OBJECT_ID", "OBJECT_TYPE", "CREATED", "LAST_DDL_TIME", "TIMESTAMP", "STATUS", "TEMPO
RARY", "GENERATED", "SECONDARY"     FROM "ET$087A00040001" KU$
2mvs15623ssvy    5910289      5910289          0       73578              0

INSERT /*+ SYS_DL_CURSOR */ INTO RELATIONAL("MACLEAN"."TCACHE") ("OWNER","OBJECT_NAME","SUBOBJECT_NAME","OBJECT_ID","DATA_OBJECT_ID","OBJECT
_TYPE","CREATED","LAST_DDL_TIME","TIMESTAMP","STATUS","TEMPORARY","GENERATED","SECONDARY") VALUES (NULL,NULL,NULL,NULL,NULL,NULL,NULL,NULL,N
ULL,NULL,NULL,NULL,NULL)
6tn47a220d34q       1167         1167          0           0              0

/* 似乎因为沿用了APPEND插入模式，插入语句的并行度并未上升 */

/* 因为Oracle不想同我们分享它的小秘密，所以awr和ash报告中都不记录外部表路径插入SQL的相关信息 */

/* 好吧，它有权保持沉默！  */

That's great!