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direct path read/read temp等待事件

发表于：2024-11-23 作者：千家信息网编辑

千家信息网最后更新 2024年11月23日，当会话从磁盘直接读取数据块到PGA(绕过SGA)时，发生direct path read/read temp等待事件，下图简要描述了这种方式的读取方式：如果I/O子系统不支持异步I/Os，那么每个等

千家信息网最后更新 2024年11月23日direct path read/read temp等待事件

当会话从磁盘直接读取数据块到PGA(绕过SGA)时，发生direct path read/read temp等待事件 ，下图简要描述了这种方式的读取方式：

如果I/O子系统不支持异步I/Os，那么每个等待对应于一个物理读请求。

direct path read/direct path read temp等待事件有三个参数，位于User I/O等待类下面

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#：要读取的数据块所在数据文件的文件号

first dba：要读取的起始数据块号

block cnt：要读取的数据块数目

导致此等待事件的原因有四个：

1.执行的sql语句含有大量排序数据 order by/group by/union/distinct/rollup等

当PGA中排序区不足时，Oracle会将排序结果写入到临时表空间中（direct path write temp）。当从临时表空间中读取排序数据时会在direct path read temp事件上等待。

2.hash join

hash在构建hash table时，将不适合位于内存中的hash partition刷出到临时表空间，随后hash partition被读回到内存时会发生direct path read temp等待事件。

3.并行查询

并行执行过程中，direct path read等待事件只与Master会话派生出的Slave会话有关，Slave会话会在direct path read事件上等待，而Master会话等待的是PX Deq。

4.全表扫描

11g后如果Oracle感觉表足够大，则会进行直接路径读取。

下面依次进行测试

一、排序引起的direct path read/write temp

session1:

SQL> alter session set workarea_size_policy=manual; --设为手动pga管理

Session altered.

SQL> alter session set sort_area_size=65536;--设置排序允许的内存大小

Session altered.

SQL> create table scott.tb_test as select * from dba_objects where rownum < 1000; --创建测试表

Table created.

session2:

SQL> oradebug setospid 7038 --设置10046事件追踪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对临时表空间直接读写的统计如下：

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

----------

此时查看追踪文件：

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

此时再查看session1对临时表空间直接读写的统计：

NAME VALUE

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

physical reads direct temporary tablespace 5

physical writes direct temporary tablespace 5

可看到，由于排序操作， Oracle对临时表空间执行了5次直接写，每次写1个block；执行了2次直接读，分别读取了4个block和1个block。

也可以在sql执行过程中，查看直接读写发生的表空间和段类型：

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

可看到占用临时表空间的确实是hash对象

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

创建测试表2：

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

Table created.

hash join前session1对临时表空间直接读写的统计如下：

NAME VALUE

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

physical reads direct temporary tablespace 225

physical writes direct temporary tablespace 718

session1执行查询：

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

----------

查看追踪文件：

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

此时session1对临时表空间直接读写的统计如下：

NAME VALUE

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

physical reads direct temporary tablespace 229

physical writes direct temporary tablespace 722

可看到， 由于排序操作，Oracle对临时表空间执行了4次直接读和4次直接写 。

三、并行查询引起的direct path read

为了效果明显，增加scott.tb_test表的记录数：

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执行查询：

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

查询Master/Slave会话：

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

可看到总共有四个session，一个parent派生出了三个child。

此时查看追踪文件，可看到parent session在进行PX Deq相关等待

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 hh34: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，如果要统计parent session在direct path read上的等待，需要用如下语句：

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的值为所有child session的累积。

四、全表扫描引起的direct path read等待事件

从11G之后，direct path read不仅可用于并行查询，在符合某些条件后，串行的全表扫描也可以利用direct path read方式来完成。

以下总结引用" 深入分析direct path read（11G）http://www.itpub.net/thread-1815281-1-1.html (出处: ITPUB论坛－中国专业的IT技术社区) "的测试结果

采用direct path read完成读取的条件：

1）表大于_small_table_threshold的参数值设置。

_small_table_threshold的单位为block。默认为db cache size的2%大小，在实例启动过程中动态决定。11GR2之前，表的大小要是_small_table_threshold参数值的5倍才会采取直接路径读取方式，11GR2后只需要满足_small_table_threshold定义的大小就会采取直接路径读取。

2）表上的脏块小于表总block数的25%

3）表中的块被cache的比例小于50%的时候

自己实际测下来，1是准的，2与3不准，暂时存疑。