ASH分析方法总结-CFANZ编程社区

gv$active_session_history会每秒钟将数据库所有节点的Active Session采样一次，而dba_hist_active_sess_history则会将gv$active_session_history里的数据每10秒采样一次并持久化保存。
基于这个特征，我们可以通过分析dba_hist_active_sess_history的Session采样情况，来定位问题发生的准确时间范围，并且可以观察每个采样点的top event和top holder。

一些容易误解含义和需要关注的列说明

SEQ#

sid和serial#相同的event并不代表是同一个event等待，有时候查看等待事件需要关注这个列，这列值相同代表相同的event上的等待。
*
SESSION_STATE

waiting代表等待，此时结合event才可以说明等待在这个等待事件上，否则则是on cpu。这个问题在v$session上会经常出现，v$session.state不是waiting时，event代表上个已经经历过的等待，需要注意区分。
*
WAIT_TIME

当等于0时，表示当前在等待。当这个值是非0值时，代表上个event的等待时间。
*
TIME_WAITED

结合session_state, 当session_states值是waiting时，代表花在等待上的时间，单位是microseconds，如果采样的数据不止一行，则在最后一行统计累积时间。
*
BLOCKING_SESSION、BLOCKING_INST_ID

阻塞者会话和阻塞者实例，对于有些等待这个值是不准的，还是要根据具体的P1和P2值的含义去计算
*
TOP_LEVEL_SQL_ID

top level调用，有时候可以根据这个列查出是什么存储过程中调用的SQL。

导出ASHDUMP

依据gv$active_session_history

create table m_ash_0728 tablespace USERS as

Select *

  From gv$active_session_history

 Where sample_time Between

       to_timestamp('2017/07/28 13:00', 'yyyy/mm/dd hh24:mi') And

       to_timestamp('2017/07/28 15:00', 'yyyy/mm/dd hh24:mi');

 

create table m_sqltext tablespace users as

Select * From gv$sql;
	*

依据dba_hist_active_sess_history

create table m_ash_0728 tablespace USERS as

Select *

  From dba_hist_active_sess_history

Where sample_time Between

       to_timestamp('2017/07/28 13:00', 'yyyy/mm/dd hh24:mi') And

       to_timestamp('2017/07/28 15:00', 'yyyy/mm/dd hh24:mi');

 

create table m_sqltext tablespace users as

Select * From dba_hist_sqltext;

导出

exp userid=\'/ as sysdba\' file=ashdata.dmp tables='m_ash_0728','m_sqltext'

导入

imp \'/ as sysdba\' file=ashdata.dmp log=ashdata.log full=y

常用确认问题流程

查看ASH时间范围

set line 160 pages 999

col min_time for a25

col max_time for a25

alter session set nls_timestamp_format='yyyy-mm-dd hh24:mi:ss.ff';

Select instance_number,

 Min(sample_time) min_time,

 Max(sample_time) max_time,

 Count(*) session_count

  From m_ash_0803

 Group By instance_number

 Order By instance_number;

确认问题发生的精确时间范围

set line 160 pages 999

col sample_time for a25

alter session set nls_timestamp_format='yyyy-mm-dd hh24:mi:ss.ff';

Select instance_number, sample_id, sample_time, Count(*) session_count

  From m_ash_0722_2 t

 Group By instance_number, sample_id, sample_time

 Order By instance_number, sample_time;

确定每个采样点的top n event

Select t.sample_id,

       t.sample_time,

       t.instance_number,

       t.event,

       t.session_state,

       t.c session_count

  From (Select t.*,

               rank() over(Partition By instance_number, sample_time Order By c Desc) r

          From (Select /*+ parallel(8) */

                 t.*,

                 Count(*) over(Partition By instance_number, sample_time, event) c,

                 row_number() over(Partition By instance_number, sample_time, event Order By 1) r1

                  From m_ash_2 t

                 Where sample_time >

                       to_timestamp('2017-07-22 01:25:01',

                                    'yyyy-mm-dd hh24:mi:ss')

                   And sample_time <

                       to_timestamp('2017-07-22 02:42:05',

                                    'yyyy-mm-dd hh24:mi:ss')) t

         Where r1 = 1) t

 Where r < 5

 Order By instance_number, sample_time, r;
这个例子是TOP 4

观察每个采样点的等待链

Select /*+ parallel(8) */

 Level                     lv,

 connect_by_isleaf         isleaf,

 connect_by_iscycle        iscycle,

 t.sample_id,

 t.sample_time,

 t.instance_number,

 t.session_id,

 t.sql_id,

 t.session_type,

 t.event,

 t.session_state,

 t.blocking_inst_id,

 t.blocking_session,

 t.blocking_session_status

  From m_ash_0722_2 t

 Where sample_time >

       to_timestamp('2017-07-22 22.02.08', 'yyyy-mm-dd hh24:mi:ss')

   And sample_time <

       to_timestamp('2017-07-22 22.32.31', 'yyyy-mm-dd hh24:mi:ss')

 Start With blocking_session Is Not Null

Connect By nocycle

 Prior sample_time = sample_time

          /*and ((prior sample_time) - sample_time between interval '-1'

          second and interval '1' second)*/

       And Prior blocking_inst_id = instance_number

       And Prior blocking_session = session_id

       And Prior blocking_session_serial# = session_serial#

 Order Siblings By sample_time;

基于上一步的原理来找出每个采样点的最终top holder

Select t.lv,

       t.iscycle,

       t.sample_id,

       t.sample_time,

       t.instance_number,

       t.session_id,

       t.sql_id,

       t.session_type,

       t.event,

       t.seq#,

       t.session_state,

       t.blocking_inst_id,

       t.blocking_session,

       t.blocking_session_status,

       t.c blocking_session_count

  From (Select t.*,

               row_number() over(Partition By instance_number, sample_time Order By c Desc) r

          From (Select t.*,

                       Count(*) over(Partition By instance_number, sample_time, session_id) c,

                       row_number() over(Partition By instance_number, sample_time, session_id Order By 1) r1

                  From (Select /*+ parallel(8) */

                         Level              lv,

                         connect_by_isleaf  isleaf,

                         connect_by_iscycle iscycle,

                         t.*

                          From m_ash_0722_2 t

                         Where sample_time >

                               to_timestamp('2017-07-22 22.02.08',

                                            'yyyy-mm-dd hh24:mi:ss')

                           And sample_time <

                               to_timestamp('2017-07-22 22.32.31',

                                            'yyyy-mm-dd hh24:mi:ss')

                         Start With blocking_session Is Not Null

                        Connect By nocycle

                         Prior sample_time = sample_time

                                  /*and ((prior sample_time) - sample_time between interval '-1'

                                  second and interval '1' second)*/

                               And Prior blocking_inst_id = instance_number

                               And Prior blocking_session = session_id

                               And Prior

                                    blocking_session_serial# = session_serial#) t

                 Where t.isleaf = 1) t

         Where r1 = 1) t

 Where r < 5

 Order By sample_time, r;

整体一个时间段的等待链

With ash As

 (Select /*+ materialize*/

   *

    From m_ash_0722_2

   Where sample_time >

         to_timestamp('2017-07-22 22:02:00', 'yyyy-mm-dd hh24:mi:ss')

     And sample_time <

         to_timestamp('2017-07-22 22:35:00', 'yyyy-mm-dd hh24:mi:ss')),

chains As

 (Select instance_number,

         session_id,

         Level lvl,

         sys_connect_by_path('inst ' || instance_number || ' ' || sql_id || ' ' ||

                             event,

                             ' -> ') path,

         connect_by_isleaf isleaf

    From ash

   Start With event = 'log file sync'

  Connect By nocycle(Prior blocking_session = session_id

                 And Prior blocking_session_serial# = session_serial#

                 And Prior blocking_inst_id = instance_number

                 And Prior sample_id = sample_id

                     /*And ((Prior sample_time) - sample_time Between Interval '-1'

                     Second And Interval '1' Second)*/))

Select instance_number,

       lpad(round(ratio_to_report(Count(*)) over() * 100) || '%', 5, ' ') "%This",

       Count(*) samples,

       path

  From chains

 Where isleaf = 1

 Group By instance_number, path

 Order By samples desc;

举例
基于ash查询enq: TX - row lock contention（类似队列锁性质的都可以用这个方法）

查看snap_id

select snap_id,

       to_char(begin_interval_time, 'yyyymmdd hh24:mi:ss') begin_interval_time

  from wrm$_snapshot

 where begin_interval_time between to_date('20151029 19', 'yyyymmdd hh24') and

       to_date('20151029 20', 'yyyymmdd hh24')

   and instance_number = 1

 order by 2;

根据snap_id, 结果按时间排序

select to_char(sample_time, 'yyyymmdd hh24:mi:ss'), event, count(*)

  from dba_hist_active_sess_history

 where snap_id = 188

   and wait_class <> 'Idle'

   and event = 'enq: TX - row lock contention'

   and instance_number = 2

 group by to_char(sample_time, 'yyyymmdd hh24:mi:ss'), event

 order by 1;

代入上一步的时间点，查询blocking_session并按count排序

select blocking_session, count(*), event

  from dba_hist_active_sess_history

 where snap_id = 188

   and to_char(sample_time, 'yyyymmdd hh24:mi:ss' = '20150612 12:00:06'

   and wait_class <> 'Idle'

   and event = 'enq: TX - row lock contention'

 group by blocking_session, event

 order by 2;

代入查到的sid，看在snap范围的blocking_session经历的event

select session_id,

       user_id,

       sql_id,

       blocking_session,

       event,

       to_char(sample_time, 'yyyymmdd hh24:mi:ss')

  from dba_hist_active_sess_history

 where snap_id = 188

   and to_char(sample_time, 'yyyymmdd hh24:mi:ss') <= '20150612 12:00:06'

   and to_char(sample_time, 'yyyymmdd hh24:mi:ss') >= '20150612 11:00:26'

   and wait_class <> 'Idle'

   and instance_number = 2

   and session_id = 2622;

发生阻塞的主要SQL

with tmp as

 (select /*+ materialize */distinct sql_id sqlid

    from dba_hist_active_sess_history

   where snap_id = 188

     and to_char(sample_time, 'yyyymmdd hh24:mi:ss') <= '20170301 08:43:45'

     and to_char(sample_time, 'yyyymmdd hh24:mi:ss') >= '20170301 08:04:39'

     and wait_class <> 'Idle'

     and instance_number = 2

     and event = 'enq: TX - row lock contention')

select distinct parsing_schema_name, sql_id, sql_text

  from v$sql

 where sql_id in (select sqlid from tmp);