模拟buffer busy waits等待事件
SQL> create table buffer_busy(id number,name varchar2(20));
表已创建。
SQL> insert into buffer_busy values(1,'robinson');
已创建 1 行。
SQL> insert into buffer_busy values(2,'robinson');
已创建 1 行。
SQL> commit;
提交完成。
session 1中
SQL> select sid from v$mystat where rownum=1;
SID
----------
150
begin
for i in 1..10000 loop
update buffer_busy set name='luoluo' where id=1;
commit;
end loop;
end;
/
session 2中
SQL> select sid from v$mystat where rownum=1;
SID
----------
147
begin
for i in 1..10000 loop
update buffer_busy set name='luoluo' where id=2;
commit;
end loop;
end;
/
session 3中运行如下查询语句
SQL> select event,sid,p1,p2,p3 from v$session_wait_history where sid in(150,147);
EVENT SID P1 P2 P3
---------------------------------------------------------------- ---------- ---------- ---------- ----------
buffer busy waits 147 5 28 1
buffer busy waits 147 5 28 1
buffer busy waits 147 5 28 1
buffer busy waits 147 2 105 29
buffer busy waits 147 2 89 27
buffer busy waits 147 5 28 1
buffer busy waits 147 5 28 1
SQL*Net message to client 147 1111838976 1 0
log file sync 147 4380 0 0
buffer busy waits 147 5 28 1
buffer busy waits 150 5 28 1
buffer busy waits 150 5 28 1
buffer busy waits 150 5 28 1
buffer busy waits 150 5 28 1
buffer busy waits 150 5 28 1
buffer busy waits 150 2 137 33
SQL*Net message to client 150 1111838976 1 0
log file sync 150 5715 0 0
buffer busy waits 150 5 28 1
buffer busy waits 150 5 28 1
SQL> select * from v$waitstat where count>0;
CLASS COUNT TIME
------------------ ---------- ----------
data block 2107 0
undo header 385 0
undo block 3 0
直接查询v$session_wait可能查不出什么，因为10000次循环很快就过了，如果想要通过v$session_wait直接查询到buffer busy waits 等待事件
可以将循环次数增加到100000次，我这里就不做了。
可以通过如下语句查询出造成该等待事件的SQL，但是如果该等待事件不是很显著，一闪而过，下面的SQL就不能查询到了，而我这个实验确实不能查询到什么
select sql_text from V$sqlarea where (address,hash_value) in (select sql_address,sql_hash_value from v$session where event like '%buffer busy%');
我们可以观察上面等待事件的参数p1,p2,p3，这里p1绝大部分为5，p2绝大部分为28，p3大部分为1，由于数据库是10g,p3代表的是v$waitstat中class列的block id，这里p3为1，说明是data block.通过如下语句找出可疑的对象
SQL> select owner,segment_name,segment_type,block_id from dba_extents where file_id=5 ;
OWNER SEGMENT_NAME SEGMENT_TYPE BLOCK_ID
------------------------------ --------------------------------------------------------------------------------- ------------------ ----------
ROBINSON TEST TABLE 9
ROBINSON ROBINSON TABLE 17
ROBINSON BUFFER_BUSY TABLE 25
我们能确定引起buffer busy waits等待事件的对象是ROBINSON.BUFFER_BUSY，因为只有buffer_busy这个表的block_id最接近28
然后我们可以通过如下查询
SQL> select sql_text from v$sqlarea where sql_text like '%buffer_busy%';
SQL_TEXT
--------------------------------------------------------------------------------------------------------------------
begin for i in 1..10000 loop update buffer_busy set name='luoluo' where id=1; commit; end loop; end;
begin for i in 1..10000 loop update buffer_busy set name='luoluo' where id=2; commit; end loop; end;
select sql_text from v$sqlarea where sql_text like '%buffer_busy%'
SQL> select sql_text from v$sqlarea where sql_text like '%BUFFER_BUSY%';
SQL_TEXT
------------------------------------------------------------------------------
select sql_text from v$sqlarea where sql_text like '%BUFFER_BUSY%'
UPDATE BUFFER_BUSY SET NAME='luoluo' WHERE ID=2
UPDATE BUFFER_BUSY SET NAME='luoluo' WHERE ID=1
到这里我们就能够发现引起buffer busy waits等待事件的SQL了。
