ORACLE _small_table_threshold与event 10949与direct path read测试 推荐

下面的测试环境:OS:WIN7 DB：11.2.0.3

当表所占的BLOCK数大于_small_table_threshold时，表就认为是大表，FTS大表操作结果是放到LRU末端，只CACHE一次，对小表的操作是放到MRU的末端。FTS大表时，是通过direct path read的方式读取数据。我们可以通过配置10949事件，使用进程通过db file scattered read的方式来读取数据。

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_small_table_threshold的值在数据库启动的时候自动配置成_db_block_buffer*0.02。

1，查看相当参数的值
SQL> @parameter_hide
SQL> set echo off

+------------------------------------------------------------------------+
| display hide parameter value                                       |
+------------------------------------------------------------------------+

Enter Search Parameter (i.e. max|all) : db_block_buffer

PARAMETER                                SESSION_VALUE        INSTANCE_VALUE       DESCRIPTION
---------------------------------------- -------------------- -------------------- ------------------------------------------------------------
_db_block_buffers                        63680                63680                Number of database blocks cached in memory: hidden parameter
SQL> @parameter_hide
SQL> set echo off

+------------------------------------------------------------------------+
| display hide parameter value                                       |
+------------------------------------------------------------------------+

Enter Search Parameter (i.e. max|all) : small

PARAMETER                                SESSION_VALUE        INSTANCE_VALUE       DESCRIPTION
---------------------------------------- -------------------- -------------------- ------------------------------------------------------------
_small_table_threshold                   1273                 1273                 lower threshold level of table size for direct reads
2，创建一个表TEST1,块的个数大小small_table_threshold
SQL> exec show_space(p_segname=>'TEST1',P_OWNER=>'SCOTT',P_TYPE=>'TABLE');
Unformatted Blocks .....................              62
FS1 Blocks (0-25)  .....................               1
FS2 Blocks (25-50) .....................               0
FS3 Blocks (50-75) .....................               0
FS4 Blocks (75-100).....................              52
Full Blocks        .....................           1,137
Total Blocks............................           1,280
Total Bytes.............................      10,485,760
Total MBytes............................              10
Unused Blocks...........................               0
Unused Bytes............................               0
Last Used Ext FileId....................               5
Last Used Ext BlockId...................         411,008
Last Used Block.........................             128

PL/SQL procedure successfully completed.
3,创建一个小表
SQL> create table scott.test2 as select * from scott.test1 where rownum<75000;

Table created.

SQL> exec show_space(p_segname=>'TEST2',P_OWNER=>'SCOTT',P_TYPE=>'TABLE');
Unformatted Blocks .....................               0
FS1 Blocks (0-25)  .....................               0
FS2 Blocks (25-50) .....................               0
FS3 Blocks (50-75) .....................               0
FS4 Blocks (75-100).....................               0
Full Blocks        .....................           1,067
Total Blocks............................           1,152
Total Bytes.............................       9,437,184
Total MBytes............................               9
Unused Blocks...........................              59
Unused Bytes............................         483,328
Last Used Ext FileId....................               5
Last Used Ext BlockId...................         413,440
Last Used Block.........................              69

PL/SQL procedure successfully completed.

4，对大表进行多次访问
我们可以看到物理读都是一样，并没有减小，说明BLOCK没有CACHE到BUFFER CACHE中
SQL> set autotrace traceonly;
SQL> select count(*) from scott.test1;

Execution Plan
----------------------------------------------------------
Plan hash value: 3896847026

--------------------------------------------------------------------
| Id  | Operation          | Name  | Rows  | Cost (%CPU)| Time     |
--------------------------------------------------------------------
|   0 | SELECT STATEMENT   |       |     1 |   349   (1)| 00:00:05 |
|   1 |  SORT AGGREGATE    |       |     1 |            |          |
|   2 |   TABLE ACCESS FULL| TEST1 | 66578 |   349   (1)| 00:00:05 |
--------------------------------------------------------------------

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

Statistics
----------------------------------------------------------
0  recursive calls
1  db block gets
1200  consistent gets
1190  physical reads
0  redo size
425  bytes sent via SQL*Net to client
415  bytes received via SQL*Net from client
2  SQL*Net roundtrips to/from client
0  sorts (memory)
0  sorts (disk)
1  rows processed

SQL> select count(*) from scott.test1;

Execution Plan
----------------------------------------------------------
Plan hash value: 3896847026

--------------------------------------------------------------------
| Id  | Operation          | Name  | Rows  | Cost (%CPU)| Time     |
--------------------------------------------------------------------
|   0 | SELECT STATEMENT   |       |     1 |   349   (1)| 00:00:05 |
|   1 |  SORT AGGREGATE    |       |     1 |            |          |
|   2 |   TABLE ACCESS FULL| TEST1 | 66578 |   349   (1)| 00:00:05 |
--------------------------------------------------------------------

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

Statistics
----------------------------------------------------------
0  recursive calls
1  db block gets
1200  consistent gets
1190  physical reads
0  redo size
425  bytes sent via SQL*Net to client
415  bytes received via SQL*Net from client
2  SQL*Net roundtrips to/from client
0  sorts (memory)
0  sorts (disk)
1  rows processed
5，下面是trace进程
查看进程是以那种方式访问数据块的
SQL> oradebug setmypid
Statement processed.
SQL> oradebug event 10046 trace name context forever,level 12;
Statement processed.
SQL> select count(*) from scott.test1;

SQL> oradebug tracefile_name
E:\APP\LUOPING\diag\rdbms\winorcl\winorcl\trace\winorcl_ora_8624.trc

PARSING IN CURSOR #262715460 len=32 dep=0 uid=0 oct=3 lid=0 tim=13695518487 hv=3636002668 ad='b88bc3dc' sqlid='gtwt4m7cbj
select count(*) from scott.test1
END OF STMT
PARSE #262715460:c=0,e=60,p=0,cr=0,cu=0,mis=0,r=0,dep=0,og=1,plh=3896847026,tim=13695518486
EXEC #262715460:c=0,e=23,p=0,cr=0,cu=0,mis=0,r=0,dep=0,og=1,plh=3896847026,tim=13695518602
WAIT #262715460: nam='SQL*Net message to client' ela= 2 driver id=1111838976 #bytes=1 p3=0 obj#=75700 tim=13695518629
WAIT #262715460: nam='direct path read' ela= 598 file number=5 first dba=409859 block cnt=13 obj#=75700 tim=13695519645
WAIT #262715460: nam='direct path read' ela= 367 file number=5 first dba=409889 block cnt=15 obj#=75700 tim=13695520603
WAIT #262715460: nam='direct path read' ela= 316 file number=5 first dba=409905 block cnt=15 obj#=75700 tim=13695521114
WAIT #262715460: nam='direct path read' ela= 136 file number=5 first dba=409921 block cnt=15 obj#=75700 tim=13695521445
WAIT #262715460: nam='direct path read' ela= 4731 file number=5 first dba=409937 block cnt=15 obj#=75700 tim=13695526371
WAIT #262715460: nam='direct path read' ela= 1603 file number=5 first dba=409986 block cnt=126 obj#=75700 tim=13695529850
WAIT #262715460: nam='direct path read' ela= 553 file number=5 first dba=410114 block cnt=126 obj#=75700 tim=13695531752
WAIT #262715460: nam='direct path read' ela= 734 file number=5 first dba=410242 block cnt=126 obj#=75700 tim=13695533842
WAIT #262715460: nam='direct path read' ela= 589 file number=5 first dba=410370 block cnt=126 obj#=75700 tim=13695535660
WAIT #262715460: nam='direct path read' ela= 1648 file number=5 first dba=410498 block cnt=126 obj#=75700 tim=13695538106
WAIT #262715460: nam='direct path read' ela= 374 file number=5 first dba=410626 block cnt=126 obj#=75700 tim=13695539715
WAIT #262715460: nam='direct path read' ela= 834 file number=5 first dba=410754 block cnt=126 obj#=75700 tim=13695541868
WAIT #262715460: nam='direct path read' ela= 634 file number=5 first dba=410882 block cnt=126 obj#=75700 tim=13695543782
这里可以看到是以direct path read的方式访问数据

7，刷新一下buffer_cache
SQL> alter system flush buffer_cache;

System altered.
8.访问小表
每一次访问小表有物理读，从2次开发物理读消失，说明BLOCK已经CACHE到BUFFER CACHE中了
SQL> select count(*) from scott.test2;

Execution Plan
----------------------------------------------------------
Plan hash value: 634289536

--------------------------------------------------------------------
| Id  | Operation          | Name  | Rows  | Cost (%CPU)| Time     |
--------------------------------------------------------------------
|   0 | SELECT STATEMENT   |       |     1 |   299   (1)| 00:00:04 |
|   1 |  SORT AGGREGATE    |       |     1 |            |          |
|   2 |   TABLE ACCESS FULL| TEST2 | 74999 |   299   (1)| 00:00:04 |
--------------------------------------------------------------------

Statistics
----------------------------------------------------------
0  recursive calls
0  db block gets
1071  consistent gets
1068  physical reads
0  redo size
425  bytes sent via SQL*Net to client
415  bytes received via SQL*Net from client
2  SQL*Net roundtrips to/from client
0  sorts (memory)
0  sorts (disk)
1  rows processed

SQL> select count(*) from scott.test2;

Execution Plan
----------------------------------------------------------
Plan hash value: 634289536

--------------------------------------------------------------------
| Id  | Operation          | Name  | Rows  | Cost (%CPU)| Time     |
--------------------------------------------------------------------
|   0 | SELECT STATEMENT   |       |     1 |   299   (1)| 00:00:04 |
|   1 |  SORT AGGREGATE    |       |     1 |            |          |
|   2 |   TABLE ACCESS FULL| TEST2 | 74999 |   299   (1)| 00:00:04 |
--------------------------------------------------------------------

Statistics
----------------------------------------------------------
0  recursive calls
0  db block gets
1071  consistent gets
0  physical reads
0  redo size
425  bytes sent via SQL*Net to client
415  bytes received via SQL*Net from client
2  SQL*Net roundtrips to/from client
0  sorts (memory)
0  sorts (disk)
1  rows processed

9，下面通过配置10949事件来改变块的访问方式
SQL> alter session set events '10949 trace name context forever, level 1';

Session altered.

SQL> select count(*) from scott.test1;

select count(*) from scott.test1
END OF STMT
PARSE #258512224:c=0,e=41,p=0,cr=0,cu=0,mis=0,r=0,dep=0,og=1,plh=3896847026,tim=13852556637
EXEC #258512224:c=0,e=24,p=0,cr=0,cu=0,mis=0,r=0,dep=0,og=1,plh=3896847026,tim=13852556706
WAIT #258512224: nam='SQL*Net message to client' ela= 1 driver id=1111838976 #bytes=1 p3=0 obj#=0 tim=13852556724
WAIT #258512224: nam='db file sequential read' ela= 551 file#=5 block#=409858 blocks=1 obj#=75700 tim=13852557314
WAIT #258512224: nam='db file scattered read' ela= 367 file#=5 block#=409859 blocks=5 obj#=75700 tim=13852557812
WAIT #258512224: nam='db file scattered read' ela= 424 file#=5 block#=409864 blocks=8 obj#=75700 tim=13852558366
WAIT #258512224: nam='db file scattered read' ela= 408 file#=5 block#=409873 blocks=7 obj#=75700 tim=13852558909
WAIT #258512224: nam='db file scattered read' ela= 521 file#=5 block#=409880 blocks=8 obj#=75700 tim=13852559557
WAIT #258512224: nam='db file scattered read' ela= 412 file#=5 block#=409889 blocks=7 obj#=75700 tim=13852560061
WAIT #258512224: nam='db file scattered read' ela= 427 file#=5 block#=409896 blocks=8 obj#=75700 tim=13852560716
WAIT #258512224: nam='db file scattered read' ela= 519 file#=5 block#=409905 blocks=7 obj#=75700 tim=13852561331
WAIT #258512224: nam='db file scattered read' ela= 528 file#=5 block#=409912 blocks=8 obj#=75700 tim=13852561948
WAIT #258512224: nam='db file scattered read' ela= 459 file#=5 block#=409921 blocks=7 obj#=75700 tim=13852562493
WAIT #258512224: nam='db file scattered read' ela= 477 file#=5 block#=409928 blocks=8 obj#=75700 tim=13852563103
WAIT #258512224: nam='db file scattered read' ela= 429 file#=5 block#=409937 blocks=7 obj#=75700 tim=13852563662
WAIT #258512224: nam='db file scattered read' ela= 470 file#=5 block#=409944 blocks=8 obj#=75700 tim=13852564261
WAIT #258512224: nam='db file scattered read' ela= 424 file#=5 block#=409953 blocks=7 obj#=75700 tim=13852564809
WAIT #258512224: nam='db file scattered read' ela= 472 file#=5 block#=409960 blocks=8 obj#=75700 tim=13852565401
WAIT #258512224: nam='db file scattered read' ela= 409 file#=5 block#=409969 blocks=7 obj#=75700 tim=13852565972
WAIT #258512224: nam='db file scattered read' ela= 426 file#=5 block#=409976 blocks=8 obj#=75700 tim=13852566498
WAIT #258512224: nam='db file scattered read' ela= 2543 file#=5 block#=409986 blocks=126 obj#=75700 tim=13852569237
WAIT #258512224: nam='db file scattered read' ela= 2434 file#=5 block#=410114 blocks=126 obj#=75700 tim=13852572799
WAIT #258512224: nam='db file scattered read' ela= 2535 file#=5 block#=410242 blocks=126 obj#=75700 tim=13852576312
WAIT #258512224: nam='db file scattered read' ela= 2641 file#=5 block#=410370 blocks=126 obj#=75700 tim=13852580912
WAIT #258512224: nam='db file sequential read' ela= 284 file#=5 block#=410881 blocks=1 obj#=75700 tim=13852582218
WAIT #258512224: nam='db file sequential read' ela= 292 file#=5 block#=409857 blocks=1 obj#=75700 tim=13852582591
WAIT #258512224: nam='db file scattered read' ela= 297 file#=5 block#=411008 blocks=2 obj#=75700 tim=13852582924
WAIT #258512224: nam='db file scattered read' ela= 2563 file#=5 block#=410498 blocks=126 obj#=75700 tim=13852585689
WAIT #258512224: nam='db file scattered read' ela= 2604 file#=5 block#=410626 blocks=126 obj#=75700 tim=13852589282
WAIT #258512224: nam='db file scattered read' ela= 2548 file#=5 block#=410754 blocks=126 obj#=75700 tim=13852592817
WAIT #258512224: nam='db file scattered read' ela= 1853 file#=5 block#=410882 blocks=78 obj#=75700 tim=13852595699
WAIT #258512224: nam='db file scattered read' ela= 1267 file#=5 block#=410960 blocks=48 obj#=75700 tim=13852597583
WAIT #258512224: nam='db file scattered read' ela= 1550 file#=5 block#=411072 blocks=64 obj#=75700 tim=13852599520

已经更改成db file scattered read了。