[20171120]11G关闭直接路径读.txt

--//今天做filesystemio_options参数测试时,遇到一个关于直接路径读的问题.
--//如果看以前的博客介绍,设置"_serial_direct_read"=never或者events '10949 trace name context forever';就可以关闭直接路径读.

--//我的测试在11GR2下set events '10949 trace name context forever';不行.
--//通过例子来说明问题

1.环境:
SCOTT@book> @ &r/ver1
PORT_STRING                    VERSION        BANNER
------------------------------ -------------- --------------------------------------------------------------------------------
x86_64/Linux 2.4.xx            11.2.0.4.0     Oracle Database 11g Enterprise Edition Release 11.2.0.4.0 - 64bit Production

SCOTT@book> create table t as select rownum id from dual connect by level<=2;
Table created.

SCOTT@book> ALTER TABLE t MINIMIZE RECORDS_PER_BLOCK ;
Table altered.
--//这样可以实现每块2条记录.

SCOTT@book> insert into t select rownum+2 from dual connect by level <=64000-2;
63998 rows created.

SCOTT@book> commit ;
Commit complete.

insert into t select * from t;
insert into t select * from t;
insert into t select * from t;
commit ;

--//分析表略.

SCOTT@book> select OWNER,SEGMENT_NAME,SEGMENT_TYPE,HEADER_FILE,HEADER_BLOCK,BYTES,BLOCKS from dba_segments where owner=user and segment_name='T';
OWNER  SEGMENT_NAME         SEGMENT_TYPE       HEADER_FILE HEADER_BLOCK      BYTES     BLOCKS
------ -------------------- ------------------ ----------- ------------ ---------- ----------
SCOTT  T                    TABLE                        4          546 2153775104     262912

--//占用2153775104/1024/1024=2054M,262912块.

SCOTT@book> select object_id,data_object_id from dba_objects where owner=user and object_name='T';
OBJECT_ID DATA_OBJECT_ID
---------- --------------
     90702          90702

2.测试1:
SCOTT@book> alter system flush buffer_cache;
System altered.
    
SCOTT@book> select count(*) from v$bh where OBJD=90702 and STATUS<>'free';
  COUNT(*)
----------
         0

SCOTT@book> select count(*) from t ;
  COUNT(*)
----------
    512000

SCOTT@book> @ &r/dpc '' ''
PLAN_TABLE_OUTPUT
-------------------------------------
SQL_ID  45vdc2q5hs1f3, child number 1
-------------------------------------
select count(*) from t
Plan hash value: 2966233522
----------------------------------------------------------------------------------------------------------------------
| Id  | Operation          | Name | Starts | E-Rows | Cost (%CPU)| E-Time   | A-Rows |   A-Time   | Buffers | Reads  |
----------------------------------------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT   |      |      1 |        | 71059 (100)|          |      1 |00:00:01.24 |     256K|    256K|
|   1 |  SORT AGGREGATE    |      |      1 |      1 |            |          |      1 |00:00:01.24 |     256K|    256K|
|   2 |   TABLE ACCESS FULL| T    |      1 |    512K| 71059   (1)| 00:14:13 |    512K|00:00:01.21 |     256K|    256K|
----------------------------------------------------------------------------------------------------------------------
Query Block Name / Object Alias (identified by operation id):
-------------------------------------------------------------
   1 - SEL$1
   2 - SEL$1 / T@SEL$1

SCOTT@book> select count(*) from v$bh where OBJD=90702 and STATUS<>'free';
  COUNT(*)
----------
        11
  
--//看reads=256K,仅仅缓存11块就能确定问题.现在是直接路径读.执行多次看执行依旧.

SCOTT@book> @ &r/viewsess "physical reads direct"
NAME                                       STATISTIC#      VALUE        SID
------------------------------------------ ---------- ---------- ----------
physical reads direct                              97     256064        274
physical reads direct temporary tablespace        110          0        274
physical reads direct (lob)                       176          0        274

3.测试set events '10949 trace name context forever';

SCOTT@book> host oerr ora 10949
10949, 00000, "Disable autotune direct path read for full table scan"
// *Cause:
// *Action:  Disable autotune direct path read for serial full table scan.
--//按照介绍应该这个也可以.

SCOTT@book> alter session set events '10949 trace name context forever';
Session altered.

SCOTT@book> alter session set statistics_level=all ;
Session altered.

SCOTT@book> alter system flush buffer_cache;
System altered.

SCOTT@book> select count(*) from v$bh where OBJD=90702 and STATUS<>'free';
  COUNT(*)
----------
         0
SCOTT@book> select count(*) from t ;
  COUNT(*)
----------
    512000

SCOTT@book> @ &r/dpc '' ''
PLAN_TABLE_OUTPUT
-------------------------------------
SQL_ID  45vdc2q5hs1f3, child number 1
-------------------------------------
select count(*) from t
Plan hash value: 2966233522
----------------------------------------------------------------------------------------------------------------------
| Id  | Operation          | Name | Starts | E-Rows | Cost (%CPU)| E-Time   | A-Rows |   A-Time   | Buffers | Reads  |
----------------------------------------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT   |      |      1 |        | 71059 (100)|          |      1 |00:00:00.71 |     256K|    256K|
|   1 |  SORT AGGREGATE    |      |      1 |      1 |            |          |      1 |00:00:00.71 |     256K|    256K|
|   2 |   TABLE ACCESS FULL| T    |      1 |    512K| 71059   (1)| 00:14:13 |    512K|00:00:00.69 |     256K|    256K|
----------------------------------------------------------------------------------------------------------------------
Query Block Name / Object Alias (identified by operation id):
-------------------------------------------------------------
   1 - SEL$1
   2 - SEL$1 / T@SEL$1
20 rows selected.

SCOTT@book> @ &r/viewsess "physical reads direct"
NAME                                       STATISTIC#      VALUE        SID
------------------------------------------ ---------- ---------- ----------
physical reads direct                              97     256064        274
physical reads direct temporary tablespace        110          0        274
physical reads direct (lob)                       176          0        274

SCOTT@book> select count(*) from v$bh where OBJD=90702 and STATUS<>'free';
  COUNT(*)
----------
        11

--//看reads=256K,仅仅缓存11块就能确定问题.现在是直接路径读.执行多次看执行依旧.
--//说明设置set events '10949 trace name context forever';无法关闭直接路径读.

4.设置"_serial_direct_read"=never;

SCOTT@book> alter session set "_serial_direct_read"=never;
Session altered.

SCOTT@book> alter session set statistics_level=all ;
Session altered.

SCOTT@book> alter system flush buffer_cache;
System altered.

SCOTT@book> select count(*) from v$bh where OBJD=90702 and STATUS<>'free';
  COUNT(*)
----------
         0
SCOTT@book> select count(*) from v$bh where OBJD=90702 and STATUS<>'free';

COUNT(*)
----------
         0

SCOTT@book> select count(*) from t ;
  COUNT(*)
----------
    512000

SCOTT@book> @ &r/dpc '' ''
PLAN_TABLE_OUTPUT
-------------------------------------
SQL_ID  45vdc2q5hs1f3, child number 1
-------------------------------------
select count(*) from t
Plan hash value: 2966233522
----------------------------------------------------------------------------------------------------------------------
| Id  | Operation          | Name | Starts | E-Rows | Cost (%CPU)| E-Time   | A-Rows |   A-Time   | Buffers | Reads  |
----------------------------------------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT   |      |      1 |        | 71059 (100)|          |      1 |00:00:01.80 |     256K|    256K|
|   1 |  SORT AGGREGATE    |      |      1 |      1 |            |          |      1 |00:00:01.80 |     256K|    256K|
|   2 |   TABLE ACCESS FULL| T    |      1 |    512K| 71059   (1)| 00:14:13 |    512K|00:00:01.78 |     256K|    256K|
----------------------------------------------------------------------------------------------------------------------
Query Block Name / Object Alias (identified by operation id):
-------------------------------------------------------------
   1 - SEL$1
   2 - SEL$1 / T@SEL$1

SCOTT@book> @ &r/viewsess "physical reads direct"
NAME                                                                   STATISTIC#      VALUE        SID
---------------------------------------------------------------------- ---------- ---------- ----------
physical reads direct                                                          97          0        274
physical reads direct temporary tablespace                                    110          0        274
physical reads direct (lob)                                                   176          0        274
--//直接路径读为0

SCOTT@book> select count(*) from v$bh where OBJD=90702 and STATUS<>'free';
  COUNT(*)
----------
     49959

--//已经缓存了49959块.多次执行如下语句:

SCOTT@book> select count(*) from t ;
  COUNT(*)
----------
    512000

SCOTT@book> @ &r/dpc '' ''
PLAN_TABLE_OUTPUT
-------------------------------------
SQL_ID  45vdc2q5hs1f3, child number 1
-------------------------------------
select count(*) from t
Plan hash value: 2966233522
----------------------------------------------------------------------------------------------------------------------
| Id  | Operation          | Name | Starts | E-Rows | Cost (%CPU)| E-Time   | A-Rows |   A-Time   | Buffers | Reads  |
----------------------------------------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT   |      |      1 |        | 71059 (100)|          |      1 |00:00:01.37 |     256K|    217K|
|   1 |  SORT AGGREGATE    |      |      1 |      1 |            |          |      1 |00:00:01.37 |     256K|    217K|
|   2 |   TABLE ACCESS FULL| T    |      1 |    512K| 71059   (1)| 00:14:13 |    512K|00:00:01.35 |     256K|    217K|
----------------------------------------------------------------------------------------------------------------------
Query Block Name / Object Alias (identified by operation id):
-------------------------------------------------------------
   1 - SEL$1
   2 - SEL$1 / T@SEL$1
20 rows selected.

--//你可以发现读的数量降到了217K块.

总结:
--//测试一定要注意一些oracle版本的变化,一些参数可能随着版本的变化发生变化.
--//这也是我为什么每次写blog都指明我当前的使用版本.
--//关闭直接路径读alter session set "_serial_direct_read"=never;对于11GR2才有效.

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