[20180828]exadata--豆腐渣系统的保护神.txt

[20180828]exadata--豆腐渣系统的保护神.txt

--//昨天看awr报表发现如下,时间8-9点报表,这个时间病房业务很少,主要门诊的业务:

1.awr报表情况:
top 10 foreground events by total wait time

event                               waits     total wait time (sec)     wait avg(ms)     % db time     wait class
db cpu                                                       10.7k                            77.9      
direct path read                1,206,717                   1745.2                 1          12.8     user i/o
cell single block physical read   371,283                    901.5                 2           6.6     user i/o
log file sync                     382,715                    626.8                 2           4.6     commit

--//exadata机器什么会存在这么大direct path read呢?难道不出现cell smart table scan.
--//1745.2*1000/1206717 = 1.44623801603855750768ms

segments by direct physical reads

    total direct physical reads: 210,215,077
    captured segments account for 100.0% of total

owner        tablespace name   object name     subobject name     obj. type     direct reads   %total
xxxxxx_yyy   xxxxxx_yyy        emr_bl03                           table         201,275,448     95.75
xxxxxx_zzz   xxxxxx_zzz        ms_cf01                            table           7,072,230     3.36

--//201275448*8192/1024/1024/1024/1024 = 1.49961894750595092773,与iostat by function/filetype summary看到的统计一致.
--//查看io统计:
iostat by function/filetype summary

'data' columns suffixed with m,g,t,p are in multiples of 1024 other columns suffixed with k,m,g,t,p are in multiples of 1000
ordered by (data read + write) desc for each function

function/file name           reads: data     reqs per sec     data per sec     writes: data     reqs per sec     data per sec     waits: count     avg tm(ms)
direct reads                 1.5t            436.59           434.062          0m                       0.00               0m     0      
direct reads (data file)     1.5t            436.59           434.062          0m                       0.00               0m     0      
smart scan                   68.5g            20.03           19.37m           0m                       0.00               0m     0      
smart scan (data file)       68.5g            20.03           19.37m           0m                       0.00               0m     0      
buffer cache reads            5.3g           125.81           1.491m           0m                       0.00               0m     382.1k                2.19
buffer cache reads (data file)5.3g           125.81           1.491m           0m                       0.00               0m     382.1k                2.19
--//direct reads到达1.5t.
--//应该与表emr_bl03,查询awr报表,找到如下sql语句:
sql_id='crzs1c9pnjqg2'

select xxxxxx_yyy.emr_bl03.*, xxxxxx_yyy.emr_bl_bl01.blmc
  from xxxxxx_yyy.emr_bl03
  left join xxxxxx_yyy.emr_bl_bl01
    on xxxxxx_yyy.emr_bl03.blbh    = xxxxxx_yyy.emr_bl_bl01.blbh
 where xxxxxx_yyy.emr_bl_bl01.brbh = '00366441';

sql ordered by user i/o wait time
resources reported for pl/sql code includes the resources used by all sql statements called by the code.
%total - user i/o time as a percentage of total user i/o wait time
%cpu - cpu time as a percentage of elapsed time
%io - user i/o time as a percentage of elapsed time
captured sql account for 9.4% of total user i/o wait time (s): 2,775
captured pl/sql account for 0.0% of total user i/o wait time (s): 2,775

user i/o time (s) executions  uio per exec (s) %total elapsed time (s) %cpu  %io   sql id        sql module sql text
...
            49.51          1         49.51       1.78           54.27  9.63  91.22 crzs1c9pnjqg2            select xxxxxx_yyy.emr_bl03.*, ...
 
--//仅仅看到1条.为什么?另外的实例也没有执行类似的语句.我估计没有使用绑定变量,在生成awr报表时漏掉这些类似的sql语句.
--//参考链接:http://blog.itpub.net/267265/viewspace-1749265/
--//实际上开发没有使用绑定变量,为什么不使用绑定变量....xxx.
--//消耗时间49秒.

2.分析:
--//首先看看执行计划:
zzzzz> @ &r/dpcawr crzs1c9pnjqg2 ''
plan_table_output
--------------------
sql_id crzs1c9pnjqg2
--------------------
select xxxxxx_yyy.emr_bl03.*,xxxxxx_yyy.emr_bl_bl01.blmc from
xxxxxx_yyy.emr_bl03  left join xxxxxx_yyy.emr_bl_bl01 on
xxxxxx_yyy.emr_bl03.blbh=xxxxxx_yyy.emr_bl_bl01.blbh where
xxxxxx_yyy.emr_bl_bl01.brbh = '00366441'
plan hash value: 40434530
----------------------------------------------------------------------------------------------------------
| id  | operation                     | name                    | e-rows |e-bytes| cost (%cpu)| e-time   |
----------------------------------------------------------------------------------------------------------
|   0 | select statement              |                         |        |       |   215k(100)|          |
|   1 |  hash join                    |                         |     19 | 27645 |   215k  (1)| 00:43:02 |
|   2 |   join filter create          | :bf0000                 |     19 |   817 |    16   (0)| 00:00:01 |
|   3 |    table access by index rowid| emr_bl_bl01             |     19 |   817 |    16   (0)| 00:00:01 |
|   4 |     index range scan          | i_emr_bl_bl01_brbh_cjsj |     19 |       |     3   (0)| 00:00:01 |
|   5 |   join filter use             | :bf0000                 |   3968k|  5343m|   215k  (1)| 00:43:01 |
|   6 |    table access storage full  | emr_bl03                |   3968k|  5343m|   215k  (1)| 00:43:01 |
----------------------------------------------------------------------------------------------------------
query block name / object alias (identified by operation id):
-------------------------------------------------------------
   1 - sel$c8875fe2
   3 - sel$c8875fe2 / emr_bl_bl01@sel$1
   4 - sel$c8875fe2 / emr_bl_bl01@sel$1
   6 - sel$c8875fe2 / emr_bl03@sel$2
note
-----
   - warning: basic plan statistics not available. these are only collected when:
       * hint 'gather_plan_statistics' is used for the statement or
       * parameter 'statistics_level' is set to 'all', at session or system level
35 rows selected.

zzzzz> @ &r/desc xxxxxx_yyy.emr_bl03
name  null?    type
----- -------- ----------------------------
wdbh  not null number(18)
zymz  not null number(2)
blbh  not null number(18)
wdlx  not null number(4)
wdnr           blob

zzzzz> select segment_name,bytes/1024/1024/1024 gb from dba_segments where segment_name='emr_bl03';
segment_name                 gb
-------------------- ----------
emr_bl03             12.2724609

zzzzz> select segment_name,bytes/1024/1024/1024 gb from dba_segments where segment_name in
(select segment_name from dba_lobs where table_name='emr_bl03');
segment_name                           gb
------------------------------ ----------
sys_lob0000087717c00005$$      102.436523

--//我以前大概测试过我们现在使用的exadata,select /*+ full(a) */ count(*) from big_table a;io最大吞吐量大约2.5gb/s.
--//(102.436523+12.2724609)/2.5 = 45.88359356,这样非常接近.

--//可以发现这个表存在blob字段,读取数据时访问blob字段时选择直接路径读.而不会出现cell smart table scan等待事件.
--//真不知道awr报表提示直接路径读1.5t如何得来的,估计像我前面提到那样漏掉类似的sql语句.

--//emr_bl03存在索引idx_emr_bl03_blbh.字段包括zymz, blbh, wdlx.不知道为什么没有选择index skip scan.

3.看看direct path read设计那些数据段:
zzzzz> select event,count(*) from dba_hist_active_sess_history where sql_id='crzs1c9pnjqg2' group by event;
event                                      count(*)
---------------------------------------- ----------
direct path read                                  5

--//5次,取样10秒1次,需要50秒.这样与前面的执行时间基本对上.主要等待事件就是direct path read.

zzzzz> @ &r/ev_name.sql 'direct path read'
    event#   event_id name                                     parameter1           parameter2           parameter3           wait_class_id wait_class# wait_class
---------- ---------- ---------------------------------------- -------------------- -------------------- -------------------- ------------- ----------- --------------------
       198 3926164927 direct path read                         file number          first dba            block cnt               1740759767           8 user i/o
       199  861319509 direct path read temp                    file number          first dba            block cnt               1740759767           8 user i/o

zzzzz> select event,p1,p2,p3,p1text,p2text,p3text from dba_hist_active_sess_history where sql_id='crzs1c9pnjqg2' ;
event            p1       p2   p3 p1text       p2text    p3text
---------------- --- -------- ---- ----------- --------- ---------
direct path read 56  2562432  128 file number  first dba block cnt
direct path read 59  3122176  128 file number  first dba block cnt
direct path read 59  3729536  128 file number  first dba block cnt
direct path read 60   287744  128 file number  first dba block cnt
direct path read 63    43392  128 file number  first dba block cnt

--//看看对应那些段.
zzzzz> column partition_name noprint
zzzzz> @ &r/which_obj 56 2562432
owner      segment_name         segment_type       tablespace_name                 extent_id    file_id   block_id      bytes     blocks relative_fno
---------- -------------------- ------------------ ------------------------------ ---------- ---------- ---------- ---------- ---------- ------------
xxxxxx_yyy emr_bl03             table              xxxxxx_yyy                            151         56    2556416   67108864       8192           56

zzzzz> @ &r/which_obj 59 3122176
owner      segment_name         segment_type       tablespace_name                 extent_id    file_id   block_id      bytes     blocks relative_fno
---------- -------------------- ------------------ ------------------------------ ---------- ---------- ---------- ---------- ---------- ------------
xxxxxx_yyy emr_bl03             table              xxxxxx_yyy                            162         59    3121664   67108864       8192           59

--//有点小小吃惊,抓取对应的都是表段,没有lob段.有点奇怪.应该有一些lob段.不理解.....

4.问题解决:
--//加入提示很快执行完成:
select /*+ index_ss(emr_bl03 ) */ xxxxxx_yyy.emr_bl03.*, xxxxxx_yyy.emr_bl_bl01.blmc
  from xxxxxx_yyy.emr_bl03
  left join xxxxxx_yyy.emr_bl_bl01
    on xxxxxx_yyy.emr_bl03.blbh    = xxxxxx_yyy.emr_bl_bl01.blbh
 where xxxxxx_yyy.emr_bl_bl01.brbh = '00366441';

sql_id  175cc74hfsn7k, child number 0
-------------------------------------
select /*+ index_ss(emr_bl03 ) */ xxxxxx_yyy.emr_bl03.*,
xxxxxx_yyy.emr_bl_bl01.blmc   from xxxxxx_yyy.emr_bl03   left join
xxxxxx_yyy.emr_bl_bl01     on xxxxxx_yyy.emr_bl03.blbh    =
xxxxxx_yyy.emr_bl_bl01.blbh  where xxxxxx_yyy.emr_bl_bl01.brbh =
'00366441'
plan hash value: 2710181240
--------------------------------------------------------------------------------------------------------
| id  | operation                    | name                    | rows  | bytes | cost (%cpu)| time     |
--------------------------------------------------------------------------------------------------------
|   0 | select statement             |                         |       |       |   130 (100)|          |
|   1 |  nested loops                |                         |    19 | 27645 |   130   (0)| 00:00:02 |
|   2 |   table access by index rowid| emr_bl_bl01             |    19 |   817 |    16   (0)| 00:00:01 |
|*  3 |    index range scan          | i_emr_bl_bl01_brbh_cjsj |    19 |       |     3   (0)| 00:00:01 |
|   4 |   table access by index rowid| emr_bl03                |     1 |  1412 |     6   (0)| 00:00:01 |
|*  5 |    index skip scan           | idx_emr_bl03_blbh       |     1 |       |     5   (0)| 00:00:01 |
--------------------------------------------------------------------------------------------------------
predicate information (identified by operation id):
---------------------------------------------------
   3 - access("emr_bl_bl01"."brbh"='00366441')
   5 - access("emr_bl03"."blbh"="emr_bl_bl01"."blbh")
       filter("emr_bl03"."blbh"="emr_bl_bl01"."blbh")
 
--//感觉很奇怪为什么oracle不选择index skip scan.而且走index skip scan 的cost的成本更低.
--//我发现zymz字段仅仅存在3个值1,2,3.没有直方图信息,会是这个原因吗?感觉不对,还是重新分析看看:

begin
  sys.dbms_stats.gather_table_stats (
     ownname           => 'xxxxxx_yyy'
    ,tabname           => 'emr_bl03'
    ,estimate_percent  => sys.dbms_stats.auto_sample_size
    ,method_opt        => 'for all columns size repeat for colunms zymz size 254'
    ,degree            => 4
    ,cascade           => true
    ,no_invalidate  => false);
end;
/

--//依旧不行.先设置索引idx_emr_bl03_blbh属性invisible,建立新索引如下:

create index xxxxxx_yyy.i_emr_bl03_blbh on xxxxxx_yyy.emr_bl03
(blbh)
logging
tablespace xxxxxx_yyy
pctfree    10
initrans   2
maxtrans   255
storage    (
            initial          64k
            next             1m
            maxsize          unlimited
            minextents       1
            maxextents       unlimited
            pctincrease      0
            buffer_pool      default
            flash_cache      default
            cell_flash_cache default
           )
noparallel;

--//测试ok,执行计划不在贴出.
--//感觉index skip scan的行为难以控制.
--//alter index xxxxxx_yyy.i_emr_bl03_blbh             invisible;
--//alter index xxxxxx_yyy.xxxxxx_yyy.i_emr_bl03_blbh  visible;

select /*+ cardinality(emr_bl_bl01 1) */ xxxxxx_yyy.emr_bl03.*, xxxxxx_yyy.emr_bl_bl01.blmc
  from xxxxxx_yyy.emr_bl03
  left join xxxxxx_yyy.emr_bl_bl01
    on xxxxxx_yyy.emr_bl03.blbh    = xxxxxx_yyy.emr_bl_bl01.blbh
 where xxxxxx_yyy.emr_bl_bl01.brbh = '00366441';

--//问题依旧不会选择跳跃索引,放弃探究...

总结:
--//实际上我想说的是正是exadata强有力的io处理能力,导致前台用户对于出现的问题根本没有反映.
--//如果是别的系统,整个io消耗殆尽.根本跑不起来....
--//最让我吃惊的是执行需要49秒,前台用户对此却毫无反应,很无语....
--//如果表存在lob类型,exadata不会出现cell smart table scan等待事件,而是走direct path read.关于这点有时间我再验证看看.

--//附上使用的测试脚本:
$ cat  which_obj.sql
define __file = &1
define __block = &2

set verify off
select * --owner,segment_name
from dba_extents
where file_id = &__file
            and &__block between block_id and block_id + blocks - 1
--            and rownum = 1
;


$ cat ev_name.sql
column name format a40
select * from v$event_name where lower(name) like lower('%&&1%');

# cat dpcawr.sql
set verify off
select * from table(dbms_xplan.display_awr(nvl('&1',null),null,null,'all allstats last peeked_binds &2 cost partition -projection -outline'));