等待事件：enq: SQ - contention

Top 10 Foreground Events by Total Wait Time

Event	Waits	Total Wait Time (sec)	Wait Avg(ms)	% DB time	Wait Class
enq: SQ - contention	259	87.7K	338548	23.3	Configuration
buffer busy waits	6,413	69.9K	10900	18.6	Concurrency
enq: HW - contention	1,780	63.5K	35669	16.9	Configuration
DB CPU		11.7K		3.1
gc buffer busy acquire	15,486	6112.9	395	1.6	Cluster
SQL*Net message from dblink	12,255,704	4795.8	0	1.3	Network
db file scattered read	771,893	2263.7	3	.6	User I/O
gc current request	3	1720.8	573601	.5	Cluster
SQL*Net break/reset to client	5,089,961	1307.7	0	.3	Application
db file sequential read	677,223	928.9	1	.2	User I/O

enq: SQ - contention

在内存上缓存(cache)范围内，调用sequence.nextval期间拥有此锁，赋予了cache+noorder 属性的sequence上发生。

select to_char(SAMPLE_TIME,'YYYY-MM-DD HH24:MI'),event,count(1) 

from dba_hist_active_sess_history where snap_id between xxx and xx

and instance_number=1  and event='latch free'

group by to_char(SAMPLE_TIME,'YYYY-MM-DD HH24:MI'),event 

order by 1;

select to_char(SAMPLE_TIME,'YYYY-MM-DD HH24:MI') st ,sql_id,event,program,p1,p2,p3 

from dba_hist_active_sess_history 

where snap_id between xxx and xxxx and instance_number=1 and program like '%LCK%';
http://blog.csdn.net/lqx0405/article/details/46332283