热点块引发的cache buffers cahins latch

热点块引发的Cache buffer Chains latch:

SQL语句即便适当进行了调优,有时也无法解决cache buffers cahins latch，若在编写SQL语句时的SQL工作方式,

只是持续扫描少数特定块,则在多个会话同时执行此SQL语句时,就会发生Hot Block引起的cache buffers chains latch争用。

测试方案如下:

1) 创建与测试相同的表和索引。

2) 即便有效扫描索引,多个会话也会反复扫描相同的块,因此发生Hot Block引起的cache buffers chains latch争用。

SQL> create or replace Procedure cbc_do_select(p_from in NUMBER,
2                                            p_to   IN  NUMBER) is
3  begin
4    -- 反复,集中扫描特定块
5    for idx in 1 .. 3000 loop
6      for x in (select id from cbc_test where id between p_from and p_to) loop
7        null;
8      end loop;
9    end loop;
10  end;
11  /

Procedure created

---同时执行20个读取工作
var job_no number;
begin
for idx in  1 .. 20 loop
dbms_job.submit(:job_no,'cbc_do_select(1000,1010);');
commit;
end loop;
end;

select sid,event,p1,p2,p3,p1raw from v$session where event='latch: cache buffers chains'
p1             p2     p3        p1raw
1	56	latch: cache buffers chains	758400976	150	0	000000002D3447D0

引发问题的SQL语句时select * from cbc_test where id between p_from and p_to,此SQL语句本身已经是优化的状态,因为通过cbc_test_idx索引,

只扫描必要范围并获取相应值。通过v$latch_children视图确认特定子latch使用是否偏多,从此可以间接判断是否是Hot Block引起的锁存器争用。

或者也可以捕捉v$session_wait 视图的PARAW列值,将反复被观察的值为锁存器地址利用。

SQL> SQL> SQL> SQL> select *
from (select addr, child#, gets, sleeps
from v$latch_children
where name = 'cache buffers chains'
order by sleeps desc)
where rownum <= 20   2    3    4    5    6  ;

ADDR	     CHILD#	  GETS	   SLEEPS
-------- ---------- ---------- ----------
2D3447D0	632	 22491	       74
2D351280	908    1843173	       15
2D341E34	575	 21975		6
2D33F498	518	 15291		3
2D34FA08	874	  4050		1
2D331E28	226	 16410		1
2D33296C	241	 12922		1
2D33A2D4	407	 12470		1
2D346438	670	  2590		1
2D3567B0       1024	  2836		0
2D3560D8       1014	  2830		0

ADDR	     CHILD#	  GETS	   SLEEPS
-------- ---------- ---------- ----------
2D35605C       1013	  3634		0
2D355EE0       1012	  1187		0
2D355E64       1011	  1806		0
2D355DE8       1010	  1455		0
2D355D6C       1009	  2011		0
2D355BF0       1008	  2615		0
2D355B74       1007	  3166		0
2D355AF8       1006	  8798		0
2D355A7C       1005	  2829		0

已选择20行。
已选择20行。

SLEEPS :
SLEEPS	NUMBER	Number of times a willing-to-wait latch request resulted in a session sleeping while waiting for the latch

从结果来看child# 632发生了锁存器争用,利用x$bh视图可以确认哪些块是Hot Block

SQL> select hladdr,
obj,
(select object_name
from dba_objects
where (data_object_id is null and object_id = x.obj)
or data_object_id = x.obj
and rownum = 1) as object_name,
dbarfil,
dbablk,
tch
from x$bh x
where hladdr in ('2D3447D0','2D341E34','2D351280')
order by hladdr, obj
2    3    4    5    6    7    8    9   10   11   12   13   14  ;

HLADDR		OBJ OBJECT_NAME 	    DBARFIL	DBABLK	      TCH
-------- ---------- -------------------- ---------- ---------- ----------
2D341E34	 37 I_OBJ2			  1	 32383		9
2D341E34	288 I_JOB_JOB			  1	  2017	       43
2D341E34	288 I_JOB_JOB			  1	  2017		1
2D341E34	288 I_JOB_JOB			  1	  2017		1
2D341E34	288 I_JOB_JOB			  1	  2017		1
2D341E34	288 I_JOB_JOB			  1	  2017		1
2D341E34	444 STATS_TARGET$		  2	   852		0
2D3447D0	 37 I_OBJ2			  1	 32375		9
2D3447D0	236 I_IDL_CHAR1 		  1	 63207		0
2D3447D0	267 SMON_SCN_TO_TIME_AUX	  2	 61809		0
2D3447D0	287 JOB$			  1	  2009		1

HLADDR		OBJ OBJECT_NAME 	    DBARFIL	DBABLK	      TCH
-------- ---------- -------------------- ---------- ---------- ----------
2D3447D0	287 JOB$			  1	  2009		2
2D3447D0	287 JOB$			  1	  2009		2
2D3447D0	287 JOB$			  1	  2009	       22
2D3447D0	287 JOB$			  1	  2009		1
2D351280	  2 ICOL$			  1	 83980	       25
2D351280	 37 I_OBJ2			  1	 44472	       10
2D351280	 40 I_OBJ5			  1	 14339		1
2D351280	 68 SYN$			  1	 40600	       14
2D351280	159 TRIGGER$			  1	 83747		1
2D351280       6213 WRH$_SQL_PLAN		  2	  4265		0
2D351280      75405 WRH$_SQLSTAT_INDEX		  2	  4498		0

HLADDR		OBJ OBJECT_NAME 	    DBARFIL	DBABLK	      TCH
-------- ---------- -------------------- ---------- ---------- ----------
2D351280      75500 CBC_TEST_IDX		  7	291882	       80

已选择23行。