read by other session导致oracle性能低

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问题背景:客户反应oracle库很慢很慢   (read by other session可以结合db file sequential read等待事件一块优化)

 

1检查等待事件:

 

set linesize 200 

col username for a15 

col event for a35 

col program for a20

col cpu_p for 99.99

 select ta.*, round(ta.cpu_time / tb.total_cpu * 100, 1) cpu_usage from (select s.username, s.program, s.event, s.sql_id, sum(trunc(m.cpu)) cpu_time, count(*) sum from v$sessmetric m, v$session s where (m.physical_reads > 100 or m.cpu > 100 or m.logical_reads > 100) and m.session_id = s.sid and m.session_serial_num = s.serial# and s.status = ‘ACTIVE‘ and username is not null group by s.username, s.program, s.event, s.sql_id order by 5 desc) ta, (select sum(cpu) total_cpu from v$sessmetric) tb where rownum < 11;

 

select event,count(1) from  v$session_wait group by event order by  2 desc;

发现 read by other session 排第一。

 

2找到read by other session的SQL,同时可以取一个AWR报告看看TOP SQL,都指向同一SQL。

 

select sid,

       s.username,

       s.program,

       s.action,

       logon_time,

       q.sql_text,

       q.SQL_FULLTEXT,

       q.sql_id

  from v$session s

  left join v$sql q on s.sql_hash_value = q.hash_value

 where s.sid in (select sid

                   from v$session_wait

                  where event in (‘read by other session‘));

 

3、执行一下SQL,看看SQL是执行计划。

 

    但是SQL很明显是走了一个错误的索引。

select count(*) as pageno

  from table1

 where targetid = :"SYS_B_0"

   and msgId in

       (select msgId from table2 where userId = :"SYS_B_1")

   and classname not in (:"SYS_B_2", :"SYS_B_3", :"SYS_B_4")

   and dateTime

 

4、错误的执行计划很可能是表统计信息不准确。经查询,果然是表2统计信息不准确。收集统计信息或者加hint解决问题。

    execute dbms_stats.gather_table_stats(ownname => ‘owner‘, tabname =>

    ‘table2‘, estimate_percent =>

    DBMS_STATS.AUTO_SAMPLE_SIZE, method_opt => ‘FOR ALL COLUMNS SIZE

    AUTO‘, cascade => TRUE);

 

备注:

read by other session这个等待事件其实是oracle IO问题一个比较常见的场景,会话a在进行把磁盘上的数据块读到内存(data buffer cache)中这个操作,

会话b,会话c 同时也请求这个数据块。因为会话a还完全读入内存(data buffer cache),就导致了b,c read by other session。所以会话a一般是db file sequential read  或 db file scattered read。

?也是一种热块现象。

 

当出现该问题如何解决?

一般出现该问题是由于sql导致的,或者是由于磁盘设备可能导致。

 

当出现该问题的时候,首先需要定位sql。

方法一:通过ash获得细粒度的报告,查看top sql statement 获得sql。

方法二:通过sql语句直接获得:

 

1、当前正在发生的问题:

select sql_fulltext from v$sql a,v$session b where a.sql_id=b.sql_id and b.event=‘read by other session‘;

 

2、历史曾经发生的

select a.sql_id,sql_fulltext from v$sql a,dba_hist_active_sess_history b where a.sql_id=b.sql_id and b.event=‘read by other session‘;

往往read by other session伴随着db file sequential read事件的出现。

 

另外可以查看涉及对象信息,此处就是p1,p2,p3

SELECT p1 "file#", p2 "block#", p3 "class#"

FROM v$session_wait WHERE event = ‘read by other session‘;

 

通过p1,p2,p3获得热点对象:

 

SELECT relative_fno, owner, segment_name, segment_type FROM dba_extents

WHERE file_id = &file

AND &block BETWEEN block_id AND block_id + blocks - 1;

 

另外,也可以 直接查看热点块的信息,如查看热点块导致的sql语句:

 

select sql_text

from v$sqltext a,

(select distinct a.owner, a.segment_name, a.segment_type

from dba_extents a,

(select dbarfil, dbablk

from (select dbarfil, dbablk from x$bh order by tch desc)

where rownum < 11) b

where a.RELATIVE_FNO = b.dbarfil

and a.BLOCK_ID <= b.dbablk

and a.block_id + a.blocks > b.dbablk) b

where a.sql_text like ‘%‘ || b.segment_name || ‘%‘

and b.segment_type = ‘TABLE‘

order by a.hash_value, a.address, a.piece;

 

查看热点块对象:

 

SELECT E.OWNER, E.SEGMENT_NAME, E.SEGMENT_TYPE

FROM DBA_EXTENTS E,

(SELECT *

FROM (SELECT ADDR, TS#, FILE#, DBARFIL, DBABLK, TCH

FROM X$BH

ORDER BY TCH DESC)

WHERE ROWNUM < 11) B

WHERE E.RELATIVE_FNO = B.DBARFIL

AND E.BLOCK_ID <= B.DBABLK

AND E.BLOCK_ID + E.BLOCKS > B.DBABLK;

 

找到sql之后需要做的就是查看执行计划,判断问题所在,并进行优化。

 

 1、对于在shared pool存在的cursor可以通过如下命令查看执行计划select * from table(dbms_xplan.display_cursor(‘sql_id‘,null,‘allstats‘));

2、对于历史可以通过查看awr信息获得: select * from table(dbms_xplan.display_awr(‘sql_id‘));

另外对于设备引起的需要查看磁盘读写信息,可以通过vmstat 2 200进行判断。

 

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