低效的SQL引发的cache buffers chains latch

1.低效的SQL

低效的SQL语句时发生cache buffers chains 锁存器争用的最重要原因。多个进程同时扫描大范围的索引或表时,可能广泛

地发生cache buffers chains latch争用。

 低效的SQL语句时怎样引起cache buffers chain latch争用的,我们可以测试进一步进行了解。测试方案如下:

1) 创建cbc_test(id,name)表,对于ID列创建cbc_test_idx索引。ID列是唯一键,是选择性非常好的列。

2)多个会话同时通过cbc_test_idx索引查询扫描cbc_test表

3)广泛的索引扫描引起不必要的缓冲区查询的增加

----创建测试表
SQL> create table cbc_test(id number,name char(100));

表已创建。

SQL>insert into cbc_test(id,name) select rownum,object_name from dba_objects;

SQL> select count(*) from cbc_test;

  COUNT(*)
----------
    217740

---创建索引
SQL> create index cbc_test_idx on cbc_test(id);

索引已创建。

---扫描表cbc_test的Procedure
create or replace Procedure cbc_do_select is
begin
  --不必要的索引扫描
  for x in (select /*+ index(cbc_test cbc_test_idx)*/
             *
              from cbc_test
             where id >= 0) loop
    null;
  end loop;
end;

---2个会话同时执行

SQL> var job_no number;
begin
for idx in 1 .. 20 loop
dbms_job.submit(:job_no,'cbc_do_select;');
commit;
end loop;
end;SQL>   2    3    4    5    6
  7  /

PL/SQL 过程已成功完成。

查看ASH信息:
530	1021634	33	15-6?? -14 09.43.26.937 é???	f73c0sc1z2y9k	latch: cache buffers chains	57	SELECT /*+ index(cbc_test cbc_test_idx)*/ * FROM CBC_TEST WHERE ID>=0
531	1021634	83	15-6?? -14 09.43.26.937 é???	f73c0sc1z2y9k	resmgr:cpu quantum		SELECT /*+ index(cbc_test cbc_test_idx)*/ * FROM CBC_TEST WHERE ID>=0
532	1021633	33	15-6?? -14 09.43.25.927 é???	f73c0sc1z2y9k	latch: cache buffers chains	57	SELECT /*+ index(cbc_test cbc_test_idx)*/ * FROM CBC_TEST WHERE ID>=0
533	1021633	83	15-6?? -14 09.43.25.927 é???	f73c0sc1z2y9k	resmgr:cpu quantum		SELECT /*+ index(cbc_test cbc_test_idx)*/ * FROM CBC_TEST WHERE ID>=0
534	1021632	33	15-6?? -14 09.43.24.907 é???	f73c0sc1z2y9k	latch: cache buffers chains	57	SELECT /*+ index(cbc_test cbc_test_idx)*/ * FROM CBC_TEST WHERE ID>=0
535	1021632	83	15-6?? -14 09.43.24.907 é???	f73c0sc1z2y9k	resmgr:cpu quantum		SELECT /*+ index(cbc_test cbc_test_idx)*/ * FROM CBC_TEST WHERE ID>=0
536	1021631	33	15-6?? -14 09.43.23.907 é???	f73c0sc1z2y9k	latch: cache buffers chains	57	SELECT /*+ index(cbc_test cbc_test_idx)*/ * FROM CBC_TEST WHERE ID>=0
537	1021631	83	15-6?? -14 09.43.23.907 é???	f73c0sc1z2y9k	resmgr:cpu quantum		SELECT /*+ index(cbc_test cbc_test_idx)*/ * FROM CBC_TEST WHERE ID>=0
538	1021630	33	15-6?? -14 09.43.22.897 é???	f73c0sc1z2y9k	latch: cache buffers chains	57	SELECT /*+ index(cbc_test cbc_test_idx)*/ * FROM CBC_TEST WHERE ID>=0
539	1021630	83	15-6?? -14 09.43.22.897 é???	f73c0sc1z2y9k	resmgr:cpu quantum		SELECT /*+ index(cbc_test cbc_test_idx)*/ * FROM CBC_TEST WHERE ID>=0
540	1021629	33	15-6?? -14 09.43.21.877 é???	f73c0sc1z2y9k	latch: cache buffers chains	57	SELECT /*+ index(cbc_test cbc_test_idx)*/ * FROM CBC_TEST WHERE ID>=0 

查看等待事件；
  SID EVENT                          P1      P2 P3  p1raw
1	33	latch: cache buffers chains	758421644	150	0	 000000002D34988C	0
2	83	latch: cache buffers chains	758421644	150	0	 000000002D34988C	16

查看latch: cache buffers chains p1 p2 p3 参数:

latch: cache buffers chains 等待事件的3 个参数
p1  代表的是内存中latch锁的地址
p2  代表闩锁号
p3   没意义

SELECT OBJ data_object_id, FILE#, DBABLK,CLASS, STATE, TCH from x$bh where HLADDR='P1RAW';

SQL> SELECT OBJ data_object_id, FILE#, DBABLK,CLASS, STATE, TCH,HLADDR from x$bh where HLADDR LIKE '%2D34988C%';

DATA_OBJECT_ID	    FILE#     DBABLK	  CLASS      STATE	  TCH HLADDR
-------------- ---------- ---------- ---------- ---------- ---------- --------
	 75499		4      72988	      1 	 1	     0                   2D34988C
	 75500		4      73454	      1 	 1	    0                    2D34988C
	 75499		7     293294	      1 	 1	    0                    2D34988C
	 75451		2      62537	      1 	 1	    2                    2D34988C

SQL> SELECT object_id,OBJECT_NAME FROM DBA_OBJECTS WHERE OBJECT_ID IN (75499,75500,75499,75451);

 OBJECT_ID OBJECT_NAME
---------- --------------------------------------------------------------------------------------------------------------------------------
     75451 WRH$_ACTIVE_SESSION_HISTORY
     75499 CBC_TEST
     75500 CBC_TEST_IDX

 HLADDR      RAW(4) Hash Chain Latch Address