當會話從磁盤直接讀取數據塊到PGA(繞過SGA)時，發生direct path read/read temp等待事件

，下圖簡要描述了這種方式的讀取方式：

如果I/O子系統不支持異步I/Os，那么每個等待對應于一個物理讀請求。

direct path read/direct path read temp等待事件有三個參數，位于User I/O等待類下面

SQL> select name,parameter1,parameter2,parameter3,wait_class from v$event_name where name like 'direct path %';

NAME? ? ? ? ? ? ? ? ? ? ? ? ? ?PARAMETER1? ? ? ? ? ?PARAMETER2? ? ? ? ? ?PARAMETER3? ? ? ? ? ?WAIT_CLASS

------------------------------ -------------------- -------------------- -------------------- --------------------

direct path sync? ? ? ? ? ? ? ?File number? ? ? ? ? Flags? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ?User I/O

direct path read? ? ? ? ? ? ? ?file number? ? ? ? ? first dba? ? ? ? ? ? block cnt? ? ? ? ? ? User I/O

direct path read temp? ? ? ? ? file number? ? ? ? ? first dba? ? ? ? ? ? block cnt? ? ? ? ? ? User I/O

direct path write? ? ? ? ? ? ? file number? ? ? ? ? first dba? ? ? ? ? ? block cnt? ? ? ? ? ? User I/O

direct path write temp? ? ? ? ?file number? ? ? ? ? first dba? ? ? ? ? ? block cnt? ? ? ? ? ? User I/O

file#：要讀取的數據塊所在數據文件的文件號

first dba：要讀取的起始數據塊號

block cnt：要讀取的數據塊數目

導致此等待事件的原因有四個：

1.執行的sql語句含有大量排序數據 order by/group by/union/distinct/rollup等

當PGA中排序區不足時，Oracle會將排序結果寫入到臨時表空間中(direct path write temp)。當從臨時表空間中讀取排序數據時會在direct path read temp事件上等待。

2.hash join

hash在構建hash table時，將不適合位于內存中的hash partition刷出到臨時表空間，隨后hash partition被讀回到內存時會發生direct path read temp等待事件。

3.并行查詢

并行執行過程中，direct path read等待事件只與Master會話派生出的Slave會話有關，Slave會話會在direct path read事件上等待，而Master會話等待的是PX Deq。

4.全表掃描

11g后如果Oracle感覺表足夠大，則會進行直接路徑讀取。

下面依次進行測試

一、排序引起的direct path read/write temp

session1:

SQL> alter session set workarea_size_policy=manual; --設為手動pga管理

Session altered.

SQL> alter session set sort_area_size=65536;--設置排序允許的內存大小

Session altered.

SQL> create table scott.tb_test as select * from dba_objects where rownum < 1000; --創建測試表

Table created.

session2:

SQL> oradebug setospid 7038 --設置10046事件追蹤session1

Oracle pid: 20, Unix process pid: 7038, image: oracle@node3 (TNS V1-V3)

SQL> oradebug event 10046 trace name context forever,level 12;

Statement processed.

SQL> oradebug tracefile_name

/opt/app/oracle/diag/rdbms/bddev1/BDDEV1/trace/BDDEV1_ora_7038.trc

排序前session1對臨時表空間直接讀寫的統計如下：

SQL>? select a.name,b."VALUE"

2? ?from v$statname a,v$sesstat b

3? ?where b.sid = 18

4? ?and a."STATISTIC#" = b."STATISTIC#"

5? ?and? a."NAME" like '%direct temp%';

NAME? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? VALUE

---------------------------------------------------------------- ----------

physical reads direct temporary tablespace? ? ? ? ? ? ? ? ? ? ? ? ? ? ?0

physical writes direct temporary tablespace? ? ? ? ? ? ? ? ? ? ? ? ? ? 0

session1：

SQL> select object_id from (select rownum rn,a.* from scott.tb_test a order by 1,2,3 desc) a where rn = 2;

OBJECT_ID

----------

46

此時查看追蹤文件：

select object_id from (select rownum rn,a.* from scott.tb_test a order by 1,2,3 desc) a where rn = 2

END OF STMT

PARSE #139877337074560:c=10940,e=16752,p=13,cr=64,cu=0,mis=1,r=0,dep=0,og=1,plh=240700742,tim=1542267932869427

EXEC #139877337074560:c=32,e=32,p=0,cr=0,cu=0,mis=0,r=0,dep=0,og=1,plh=240700742,tim=1542267932869711

WAIT #139877337074560: nam='SQL*Net message to client' ela= 4 driver id=1650815232 #bytes=1 p3=0 obj#=88859 tim=1542267932869799

WAIT #139877337074560: nam='Disk file operations I/O' ela= 39 FileOperation=2 fileno=201 filetype=2 obj#=88859 tim=1542267932870591

WAIT #139877337074560: nam='

direct path write temp

' ela= 5112 file number=201 first dba=6528

block cnt=1

obj#=88859 tim=1542267932875922

WAIT #139877337074560: nam='

direct path write temp

' ela= 7399 file number=201 first dba=6529

block cnt=1

obj#=88859 tim=1542267932883416

WAIT #139877337074560: nam=

'direct path write temp'

ela= 237 file number=201 first dba=6530

block cnt=1

obj#=88859 tim=1542267932883751

WAIT #139877337074560: nam='

direct path write temp'

ela= 242 file number=201 first dba=6531

block cnt=1

obj#=88859 tim=1542267932884205

WAIT #139877337074560: nam='

direct path write temp'

ela= 246 file number=201 first dba=6532

block cnt=1

obj#=88859 tim=1542267932884637

WAIT #139877337074560: nam=

'direct path read temp'

ela= 21 file number=201 first dba=6528

block cnt=4

obj#=88859 tim=1542267932884757

FETCH #139877337074560:c=1865,e=14945,p=4,cr=16,cu=2,mis=0,r=1,dep=0,og=1,plh=240700742,tim=1542267932884816

WAIT #139877337074560: nam='SQL*Net message from client' ela= 197 driver id=1650815232 #bytes=1 p3=0 obj#=88859 tim=1542267932885067

WAIT #139877337074560: nam='

direct path read temp'

ela= 16 file number=201 first dba=6532

block cnt=1

obj#=88859 tim=1542267932885400

FETCH #139877337074560:c=516,e=515,p=1,cr=0,cu=0,mis=0,r=0,dep=0,og=1,plh=240700742,tim=1542267932885620

STAT #139877337074560 id=1 cnt=1 pid=0 pos=1 obj=0 op='VIEW? (cr=16 pr=5 pw=5 time=14948 us cost=73 size=25974 card=999)'

STAT #139877337074560 id=2 cnt=999 pid=1 pos=1 obj=0 op='SORT ORDER BY (cr=16 pr=5 pw=5 time=16680 us cost=73 size=206793 card=999)'

STAT #139877337074560 id=3 cnt=999 pid=2 pos=1 obj=0 op='COUNT? (cr=16 pr=0 pw=0 time=4019 us)'

STAT #139877337074560 id=4 cnt=999 pid=3 pos=1 obj=88861 op='TABLE ACCESS FULL TB_TEST (cr=16 pr=0 pw=0 time=1268 us cost=6 size=206793 card=999)'

WAIT #139877337074560: nam='SQL*Net message to client' ela= 2 driver id=1650815232 #bytes=1 p3=0 obj#=88859 tim=1542267932885761

此時再查看session1對臨時表空間直接讀寫的統計：

NAME? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? VALUE

---------------------------------------------------------------- ----------

physical reads direct temporary tablespace? ? ? ? ? ? ? ? ? ? ? ? ? ? ?5

physical writes direct temporary tablespace? ? ? ? ? ? ? ? ? ? ? ? ? ? 5

可看到，由于排序操作，

Oracle對臨時表空間執行了5次直接寫，每次寫1個block；執行了2次直接讀，分別讀取了4個block和1個block。

也可以在sql執行過程中，查看直接讀寫發生的表空間和段類型：

SQL> select a.event,a.sid, c.sql_hash_value hash_value,decode(d.ktssosegt,1,'SORT', 2,'HASH',3,'DATA',4,'INDEX',5,'LOB_DATA',6,'LOB_INDEX',null) as segment_type,b.tablespace_name,b.file_name

2? from? ?v$session_wait a, dba_data_files b, v$session c, x$ktsso d

3? where? c.saddr? = d.ktssoses(+)

4? and c.serial#= d.ktssosno(+)

5? and d.inst_id(+) = userenv('instance')

6? and a.sid = c.sid

7? and a.p1? = b.file_id

8? and a.event like 'direct path %'

9? union all

10? select a.event,a.sid,d.sql_hash_value hash_value,decode(e.ktssosegt,1,'SORT', 2,'HASH',3,'DATA',4,'INDEX',5,'LOB_DATA',6,'LOB_INDEX',null) as segment_type,b.tablespace_name,b.file_name

11? from? ?v$session_wait a, dba_temp_files b, v$parameter c, v$session d, x$ktsso e

12? where? d.saddr? = e.ktssoses(+)

13? and d.serial#= e.ktssosno(+)

14? and e.inst_id(+) = userenv('instance')

15? and a.sid = d.sid

16? and b.file_id= a.p1 - c.value

17? and c.name= 'db_files'

18? and a.event like 'direct path %'

19? order by 1,2;

EVENT? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ?SID HASH_VALUE SEGMENT_T TABLESPACE_NAME? ? ? FILE_NAME

------------------------------ ---------- ---------- --------- -------------------- --------------------------------------------------

direct path write temp? ? ? ? ? ? ? ? 133? 474626757 HASH? ? ? TEMP? ? ? ? ? ? ? ? ?/opt/app/oracle/oradata/BDDEV1/temp01.dbf

SQL> /

EVENT? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ?SID HASH_VALUE SEGMENT_T TABLESPACE_NAME? ? ? FILE_NAME

------------------------------ ---------- ---------- --------- -------------------- --------------------------------------------------

direct path read temp? ? ? ? ? ? ? ? ?133? 474626757 HASH? ? ? TEMP? ? ? ? ? ? ? ? ?/opt/app/oracle/oradata/BDDEV1/temp01.dbf

可看到占用臨時表空間的確實是hash對象

二、hash join引起的direct path read/write temp

創建測試表2：

SQL> create table scott.tb_test2 as select * from dba_objects where rownum < 2;

Table created.

hash join前session1對臨時表空間直接讀寫的統計如下：

NAME? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? VALUE

---------------------------------------------------------------- ----------

physical reads direct temporary tablespace? ? ? ? ? ? ? ? ? ? ? ? ? ? ? 225

physical writes direct temporary tablespace? ? ? ? ? ? ? ? ? ? ? ? ? ? ?718

session1執行查詢：

SQL> select /*+ use_hash(a,b) leading(a,b)*/ a.object_id

2? ?from scott.tb_test a,scott.tb_test2 b

3? ?where a.object_name = b.object_name;

OBJECT_ID

----------

20

查看追蹤文件：

PARSING IN CURSOR #139877337073608 len=125 dep=0 uid=0 oct=3 lid=0 tim=1542269239154410 hv=3044439960 ad='a54bb978' sqlid='djdqhmuurcxws'

select /*+ use_hash(a,b) leading(a,b)*/ a.object_id

from scott.tb_test a,scott.tb_test2 b

where a.object_name = b.object_name

END OF STMT

PARSE #139877337073608:c=83,e=83,p=0,cr=0,cu=0,mis=0,r=0,dep=0,og=1,plh=3759002927,tim=1542269239154407

EXEC #139877337073608:c=53,e=54,p=0,cr=0,cu=0,mis=0,r=0,dep=0,og=1,plh=3759002927,tim=1542269239154772

WAIT #139877337073608: nam='SQL*Net message to client' ela= 3 driver id=1650815232 #bytes=1 p3=0 obj#=88864 tim=1542269239154834

WAIT #139877337073608: nam='

direct path write temp'

ela= 6823 file number=201 first dba=6528 block cnt=1 obj#=88864 tim=1542269239162866

WAIT #139877337073608: nam=

'direct path write temp'

ela= 7342 file number=201 first dba=6529 block cnt=1 obj#=88864 tim=1542269239170355

WAIT #139877337073608: nam='

direct path write temp'

ela= 249 file number=201 first dba=6530 block cnt=1 obj#=88864 tim=1542269239170970

WAIT #139877337073608: nam=

'direct path write temp'

ela= 271 file number=201 first dba=6531 block cnt=1 obj#=88864 tim=1542269239171450

WAIT #139877337073608: nam='

direct path read temp'

ela= 19 file number=201 first dba=6531 block cnt=1 obj#=88864 tim=1542269239171572

WAIT #139877337073608: nam=

'direct path read temp'

ela= 14 file number=201 first dba=6528 block cnt=1 obj#=88864 tim=1542269239171641

WAIT #139877337073608: nam=

'direct path read temp'

ela= 13 file number=201 first dba=6529 block cnt=1 obj#=88864 tim=1542269239171688

FETCH #139877337073608:c=2374,e=16849,p=3,cr=45,cu=0,mis=0,r=1,dep=0,og=1,plh=3759002927,tim=1542269239171729

WAIT #139877337073608: nam='SQL*Net message from client' ela= 170 driver id=1650815232 #bytes=1 p3=0 obj#=88864 tim=1542269239172124

WAIT #139877337073608: nam='

direct path read temp'

ela= 15 file number=201 first dba=6530 block cnt=1 obj#=88864 tim=1542269239172206

FETCH #139877337073608:c=140,e=139,p=1,cr=0,cu=0,mis=0,r=0,dep=0,og=1,plh=3759002927,tim=1542269239172308

STAT #139877337073608 id=1 cnt=1 pid=0 pos=1 obj=0 op='HASH JOIN? (cr=45 pr=4 pw=4 time=16850 us cost=55 size=145 card=1)'

STAT #139877337073608 id=2 cnt=2999 pid=1 pos=1 obj=88866 op='TABLE ACCESS FULL TB_TEST (cr=42 pr=0 pw=0 time=4799 us cost=14 size=236921 card=2999)'

STAT #139877337073608 id=3 cnt=1 pid=1 pos=2 obj=88862 op='TABLE ACCESS FULL TB_TEST2 (cr=3 pr=0 pw=0 time=29 us cost=3 size=66 card=1)'

WAIT #139877337073608: nam='SQL*Net message to client' ela= 2 driver id=1650815232 #bytes=1 p3=0 obj#=88864 tim=1542269239172458

此時session1對臨時表空間直接讀寫的統計如下：

NAME? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? VALUE

---------------------------------------------------------------- ----------

physical reads direct temporary tablespace? ? ? ? ? ? ? ? ? ? ? ? ? ? ? 229

physical writes direct temporary tablespace? ? ? ? ? ? ? ? ? ? ? ? ? ? ?722

可看到，

由于排序操作，Oracle對臨時表空間執行了4次直接讀和4次直接寫

。

三、并行查詢引起的direct path read

為了效果明顯，增加scott.tb_test表的記錄數：

SQL> insert into scott.tb_test select * from scott.tb_test;

...

修改scott.tb_test表的并行度：

SQL> alter table scott.tb_test parallel 3;

session1執行查詢：

SQL> select count(1) from scott.tb_test;

查詢Master/Slave會話：

SQL> select decode(a.qcserial#, null, 'PARENT', 'CHILD') stmt_level,a.sid,a.serial#,b.username,b.osuser,b.sql_hash_value,b.sql_address,a.degree,a.req_degree

2? from? ?v$px_session a, v$session b

3? where? a.sid = b.sid

4? order by a.qcsid, stmt_level desc;

STMT_L? ? ? ? SID? ? SERIAL# USERNAME? ? ? ? ? ? ? ? ? ? ? ?OSUSER? ? ? ? ? ? ? ? ? ? ? ? ?SQL_HASH_VALUE SQL_ADDRESS? ? ? ? ? DEGREE REQ_DEGREE

------ ---------- ---------- ------------------------------ ------------------------------ -------------- ---------------- ---------- ----------

PARENT? ? ? ? ?18? ? ? 27857 SYS? ? ? ? ? ? ? ? ? ? ? ? ? ? oracle? ? ? ? ? ? ? ? ? ? ? ? ? ? ? 305472416 0000000069D6DD30

CHILD? ? ? ? ? 20? ? ? 12579 SYS? ? ? ? ? ? ? ? ? ? ? ? ? ? oracle? ? ? ? ? ? ? ? ? ? ? ? ? ? ? 305472416 0000000069D6DD30? ? ? ? ? 3? ? ? ? ? 3

CHILD? ? ? ? ?141? ? ? 14701 SYS? ? ? ? ? ? ? ? ? ? ? ? ? ? oracle? ? ? ? ? ? ? ? ? ? ? ? ? ? ? 305472416 0000000069D6DD30? ? ? ? ? 3? ? ? ? ? 3

CHILD? ? ? ? ?149? ? ? 21869 SYS? ? ? ? ? ? ? ? ? ? ? ? ? ? oracle? ? ? ? ? ? ? ? ? ? ? ? ? ? ? 305472416 0000000069D6DD30? ? ? ? ? 3? ? ? ? ? 3

可看到總共有四個session，一個parent派生出了三個child。

此時查看追蹤文件，可看到parent session在進行PX Deq相關等待

select count(1) from scott.tb_test

END OF STMT

PARSE #139877336651544:c=19135,e=59969,p=8,cr=128,cu=0,mis=1,r=0,dep=0,og=1,plh=291292600,tim=1542272658766051

WAIT #139877336651544: nam='os thread startup' ela= 16334 p1=0 p2=0 p3=0 obj#=88866 tim=1542272658783336

WAIT #139877336651544: nam='os thread startup' ela= 15250 p1=0 p2=0 p3=0 obj#=88866 tim=1542272658798803

WAIT #139877336651544: nam='os thread startup' ela= 15279 p1=0 p2=0 p3=0 obj#=88866 tim=1542272658814265

WAIT #139877336651544: nam='PX Deq: Join ACK' ela= 3495 sleeptime/senderid=268500992 passes=3 p3=2778333136 obj#=88866 tim=1542272658817981

WAIT #139877336651544: nam='PX Deq: Join ACK' ela= 2 sleeptime/senderid=0 passes=0 p3=0 obj#=88866 tim=1542272658818053

WAIT #139877336651544: nam='PX Deq: Join ACK' ela= 2 sleeptime/senderid=0 passes=0 p3=0 obj#=88866 tim=1542272658818074

WAIT #139877336651544: nam='PX Deq: Parse Reply' ela= 4166 sleeptime/senderid=200 passes=1 p3=0 obj#=88866 tim=1542272658822383

WAIT #139877336651544: nam='PX Deq: Parse Reply' ela= 155 sleeptime/senderid=200 passes=1 p3=0 obj#=88866 tim=1542272658822593

WAIT #139877336651544: nam='PX Deq: Parse Reply' ela= 3778 sleeptime/senderid=200 passes=1 p3=0 obj#=88866 tim=1542272658826408

EXEC #139877336651544:c=1759,e=60216,p=0,cr=43,cu=0,mis=0,r=0,dep=0,og=1,plh=291292600,tim=1542272658826648

WAIT #139877336651544: nam='SQL*Net message to client' ela= 3 driver id=1650815232 #bytes=1 p3=0 obj#=88866 tim=1542272658826714

WAIT #139877336651544: nam='PX Deq: Execute Reply' ela= 5524 sleeptime/senderid=200 passes=1 p3=0 obj#=88866 tim=1542272658832302

WAIT #139877336651544: nam='PX Deq: Execute Reply' ela= 3 sleeptime/senderid=0 passes=0 p3=0 obj#=88866 tim=1542272658832414

WAIT #139877336651544: nam='PX Deq: Execute Reply' ela= 4277 sleeptime/senderid=200 passes=2 p3=0 obj#=88866 tim=1542272658836731

查看v$active_session_history，可看到派生出的child session在direct path read事件上等待：

SQL> select to_char(sample_time,'yyyymmdd hh24:mi:ss') stime,session_id,session_serial#,sql_id,session_state,event,p1,p2,p3,blocking_session

2? from v$active_session_history

3? where sample_time >= sysdate-1/24/60

4? order by 1 desc;

STIME? ? ? ? ? ? ?SESSION_ID SESSION_SERIAL# SQL_ID? ? ? ? SESSION EVENT? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? P1? ? ? ? ?P2? ? ? ? ?P3 BLOCKING_SESSION

----------------- ---------- --------------- ------------- ------- ------------------------------ ---------- ---------- ---------- ----------------

20181115 18:23:49? ? ? ? ?20? ? ? ? ? ?12581 3hhzygn93a8x0 WAITING direct path read? ? ? ? ? ? ? ? ? ? ? ? 4? ? ?116736? ? ? ? 128

20181115 18:23:49? ? ? ? 141? ? ? ? ? ?14711 3hhzygn93a8x0 WAITING direct path read? ? ? ? ? ? ? ? ? ? ? ? 4? ? ?264192? ? ? ? 128

20181115 18:23:49? ? ? ? 149? ? ? ? ? ?21927 3hhzygn93a8x0 WAITING direct path read? ? ? ? ? ? ? ? ? ? ? ? 4? ? ?275328? ? ? ? 128

20181115 18:23:48? ? ? ? ?20? ? ? ? ? ?12581 3hhzygn93a8x0 WAITING direct path read? ? ? ? ? ? ? ? ? ? ? ? 4? ? ?190848? ? ? ? 128

20181115 18:23:48? ? ? ? 141? ? ? ? ? ?14711 3hhzygn93a8x0 WAITING direct path read? ? ? ? ? ? ? ? ? ? ? ? 4? ? ? 37248? ? ? ? 128

20181115 18:23:48? ? ? ? 149? ? ? ? ? ?21927 3hhzygn93a8x0 WAITING direct path read? ? ? ? ? ? ? ? ? ? ? ? 4? ? ?205440? ? ? ? 128

20181115 18:23:47? ? ? ? ?20? ? ? ? ? ?12581 3hhzygn93a8x0 WAITING direct path read? ? ? ? ? ? ? ? ? ? ? ? 4? ? ?139776? ? ? ? 128

20181115 18:23:47? ? ? ? 141? ? ? ? ? ?14711 3hhzygn93a8x0 WAITING direct path read? ? ? ? ? ? ? ? ? ? ? ? 4? ? ?146816? ? ? ? 128

20181115 18:23:47? ? ? ? 149? ? ? ? ? ?21927 3hhzygn93a8x0 WAITING direct path read? ? ? ? ? ? ? ? ? ? ? ? 4? ? ? 56192? ? ? ? 128

...

由于parent session并不在direct path read上等待，因此，如果要統計parent session在direct path read上的等待，需要用如下語句：

SQL> select a.name, b.sid, b.value, round((sysdate - c.logon_time) * 24) hours_connected

2? from? ?v$statname a, v$sesstat b, v$session c

3? where? b.sid = c.sid

4? and a.statistic# = b.statistic#

5? and b.value? > 0

6? and a.name= 'physical reads direct'

7? order by b.value;

NAME? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? SID? ? ? VALUE HOURS_CONNECTED

------------------------------ ---------- ---------- ---------------

physical reads direct? ? ? ? ? ? ? ? ? 18? ? ?977016? ? ? ? ? ? ? ?0

其中panret session的值為所有child session的累積。

四、全表掃描引起的direct path read等待事件

從11G之后，direct path read不僅可用于并行查詢，在符合某些條件后，串行的全表掃描也可以利用direct path read方式來完成。

以下總結引用“

深入分析direct path read(11G)http://www.itpub.net/thread-1815281-1-1.html (出處: ITPUB論壇－中國專業的IT技術社區)

”的測試結果

采用direct path read完成讀取的條件：

1)表大于_small_table_threshold的參數值設置。

_small_table_threshold的單位為block。默認為db cache size的2%大小，在實例啟動過程中動態決定。11GR2之前，表的大小要是_small_table_threshold參數值的5倍才會采取直接路徑讀取方式，11GR2后只需要滿足_small_table_threshold定義的大小就會采取直接路徑讀取。

2)表上的臟塊小于表總block數的25%

3)表中的塊被cache的比例小于50%的時候

自己實際測下來，1是準的，2與3不準，暫時存疑。

關閉

direct path read

特性

1._

serial_direct_read

=false

2.Event: ?'10949 trace name context forever, level 1'

總結

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