模拟cache buffers chains latch实验

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1。模拟思路：

原理上来说，buffer cache中block的header是被放置到hash chains上，而hash chains又是放在hash bucket中，多个hash bucket被一个cache buffers chains latch保护。当多个session并发访问同一个数据块上的数据，每个session都要首先获得cache buffers chains latch，这样将造成cache buffers chains latch的争用。

2。试验步骤

（1）创建测试表big1，并创建索引

SQL> create table big1 as select * from sys.dba_objects;

Table created.

SQL> create index idx_big1 on big1(object_id);

Index created.

（2）打开5个session，各自执行如下过程：

declare
r integer;
begin
for i in 1 .. 3000000 loop
select object_id into r from big1 where object_id=97;
dbms_lock.sleep(0.001);
end loop;
end;
/

（3）执行同时查看相应的等待事件：

select spid,
       a.sid,
       d.SQL_ID,
       d.SQL_TEXT,
       d.SQL_FULLTEXT,
       a.EVENT
from v$session a, v$process b, v$sesstat c, v$sqlarea d
where c.statistic# = 12
   and c.sid = a.sid
   and a.paddr = b.addr
   and a.status = 'ACTIVE'
   AND A.TYPE = 'USER'
   and a.SQL_ID = d.SQL_ID /* and SQL_TEXT like 'SELECT * FROM( SELECT X.* , ROWNUM RN%'*/
order by value desc;

SPID SID SQL_ID SQL_TEXT SQL_FULLTEXT EVENT

1 535 718 4fpd5mb7pqmqw SELECT OBJECT_ID FROM BIG1 WHERE OBJECT_ID=97 <CLOB> latch: cache buffers chains
2 543 654 4fpd5mb7pqmqw SELECT OBJECT_ID FROM BIG1 WHERE OBJECT_ID=97 <CLOB> latch: cache buffers chains
3 539 582 4fpd5mb7pqmqw SELECT OBJECT_ID FROM BIG1 WHERE OBJECT_ID=97 <CLOB> latch: cache buffers chains
4 541 581 4fpd5mb7pqmqw SELECT OBJECT_ID FROM BIG1 WHERE OBJECT_ID=97 <CLOB> latch: cache buffers chains

可见，出现大量的cache buffers chains的latch free等待事件。