oracle direct path read temp,direct path read/read temp等待事件
當(dāng)會(huì)話從磁盤直接讀取數(shù)據(jù)塊到PGA(繞過SGA)時(shí)，發(fā)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(tǒng)不支持異步I/Os，那么每個(gè)等待對(duì)應(yīng)于一個(gè)物理讀請(qǐng)求。
direct path read/direct path read temp等待事件有三個(gè)參數(shù)，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(shù)據(jù)塊所在數(shù)據(jù)文件的文件號(hào)
first dba：要讀取的起始數(shù)據(jù)塊號(hào)
block cnt：要讀取的數(shù)據(jù)塊數(shù)目
導(dǎo)致此等待事件的原因有四個(gè)：
1.執(zhí)行的sql語句含有大量排序數(shù)據(jù) order by/group by/union/distinct/rollup等
當(dāng)PGA中排序區(qū)不足時(shí)，Oracle會(huì)將排序結(jié)果寫入到臨時(shí)表空間中(direct path write temp)。當(dāng)從臨時(shí)表空間中讀取排序數(shù)據(jù)時(shí)會(huì)在direct path read temp事件上等待。
2.hash join
hash在構(gòu)建hash table時(shí)，將不適合位于內(nèi)存中的hash partition刷出到臨時(shí)表空間，隨后hash partition被讀回到內(nèi)存時(shí)會(huì)發(fā)生direct path read temp等待事件。
3.并行查詢
并行執(zhí)行過程中，direct path read等待事件只與Master會(huì)話派生出的Slave會(huì)話有關(guān)，Slave會(huì)話會(huì)在direct path read事件上等待，而Master會(huì)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(huì)進(jìn)行直接路徑讀取。
下面依次進(jìn)行測試
一、排序引起的direct path read/write temp
session1:
SQL> alter session set workarea_size_policy=manual; --設(shè)為手動(dòng)pga管理
Session altered.
SQL> alter session set sort_area_size=65536;--設(shè)置排序允許的內(nèi)存大小
Session altered.
SQL> create table scott.tb_test as select * from dba_objects where rownum < 1000; --創(chuàng)建測試表
Table created.
session2:
SQL> oradebug setospid 7038 --設(shè)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(duì)臨時(shí)表空間直接讀寫的統(tǒng)計(jì)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(shí)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(shí)再查看session1對(duì)臨時(shí)表空間直接讀寫的統(tǒng)計(jì)：
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(duì)臨時(shí)表空間執(zhí)行了5次直接寫，每次寫1個(gè)block；執(zhí)行了2次直接讀，分別讀取了4個(gè)block和1個(gè)block。
也可以在sql執(zhí)行過程中，查看直接讀寫發(fā)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(shí)表空間的確實(shí)是hash對(duì)象
二、hash join引起的direct path read/write temp
創(chuàng)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(duì)臨時(shí)表空間直接讀寫的統(tǒng)計(jì)如下：
NAME? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? VALUE
---------------------------------------------------------------- ----------
physical reads direct temporary tablespace? ? ? ? ? ? ? ? ? ? ? ? ? ? ? 225
physical writes direct temporary tablespace? ? ? ? ? ? ? ? ? ? ? ? ? ? ?718
session1執(zhí)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(shí)session1對(duì)臨時(shí)表空間直接讀寫的統(tǒng)計(jì)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(duì)臨時(shí)表空間執(zhí)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(shù)：
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(zhí)行查詢：
SQL> select count(1) from scott.tb_test;
查詢Master/Slave會(huì)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(gè)session，一個(gè)parent派生出了三個(gè)child。
此時(shí)查看追蹤文件，可看到parent session在進(jìn)行PX Deq相關(guān)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(tǒng)計(jì)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(jié)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(yè)的IT技術(shù)社區(qū))
”的測試結(jié)果
采用direct path read完成讀取的條件：
1)表大于_small_table_threshold的參數(shù)值設(shè)置。
_small_table_threshold的單位為block。默認(rèn)為db cache size的2%大小，在實(shí)例啟動(dòng)過程中動(dòng)態(tài)決定。11GR2之前，表的大小要是_small_table_threshold參數(shù)值的5倍才會(huì)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(huì)采取直接路徑讀取。
2)表上的臟塊小于表總block數(shù)的25%
3)表中的塊被cache的比例小于50%的時(shí)候
自己實(shí)際測下來，1是準(zhǔn)的，2與3不準(zhǔn)，暫時(shí)存疑。
關(guān)閉
direct path read
特性
1._
serial_direct_read
=false
2.Event: ?'10949 trace name context forever, level 1'
總結(jié)
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