latch: row cache objects

如果有latch: row cache objects等待事件，那么会导致CPU很高，主机和数据库响应都很慢。和Rowcache及其相关Latch的视图主要有：v$latch_children,v$rowcache.

Row cache objects latch用来保护数据字典缓冲区的，进程在装载、引用或者清除数据字典缓冲区中的对象时必须获得该latch。Latch不会造成阻塞，只会导致等待。 阻塞是一种系统设计上的问题，等待是一种系统资源争用的问题。

Row cache objects latch: 当用户进程访问缓存的数据字典数值时，将使用Row cache objects latch。

分析用的SQL:

select addr,latch#,child#,level#,name,gets, misses, sleeps
from v$latch_children
where name='row cache objects' and gets<>0 order by gets;

select "WHERE", sleep_count, location
from v$latch_misses
where parent_name='row cache objects'
and sleep_count>0;

select distinct
r.cache#,
r.parameter    name,
r.type,
r.subordinate#,
r.gets
from v$rowcache r
order by 1, 4, 5;

select parameter,sum("COUNT"),sum(usage),sum(gets),sum(getmisses),sum(scans),sum(scanmisses),sum(modifications),sum(dlm_requests),sum(dlm_conflicts),sum(dlm_releases)
from v$rowcache
group by parameter;

alter session set events 'immediate trace name row_cache level level# ';