Latch: cache buffer chains等待时间的含义：

Latch: cache buffer chains等待时间的含义：

“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。”

一、查询各等待事件的含义：

search@WEBDB>r 1 select name, parameter1, parameter2, parameter3 2 from v$event_name 3* where name like 'latch%' NAME PARAMETER1 PARAMETER2 PARAMETER3

 -------------------------------------------------- ---------- ---------- ----------

latch: cache buffers chains address number tries 

latch: redo writing address number tries 

latch: redo copy address number tries

latch: Undo Hint Latch address number tries

latch: In memory undo latch address number tries 

latch: MQL Tracking Latch address number tries

latch: row cache objects address number tries 

latch: shared pool address number tries latch: library cache address number tries 

latch: library cache lock address number tries 

latch: library cache pin address number tries latch activity address number process# latch free address number tries latch: session allocation address number tries 

latch: messages address number tries latch: enqueue hash chains address number tries 

latch: ges resource hash list address number tries 

latch: gcs resource hash address number tries 

latch: cache buffers lru chain address number tries 

latch: checkpoint queue latch address number tries 

latch: cache buffer handles address number tries 

latch: object queue header operation address number tries 

latch: object queue header heap address number tries 

latch: redo allocation address number tries 

latch: KCL gc element parent latch address number tries 

latch: undo global data address number tries 

latch: Change Notification Hash table latch address number tries 

latch: virtual circuit queues address number tries 

latch: parallel query alloc buffer address number tries

二、查询等待相关的sql语句

select sql_text, sql_fulltext, t2.sid, t2.event, t2.wait_class, t2.p1, t2.p2, t2.p3, t2.state 

from v$sqlstats t1 inner join v$session t2 on t1.SQL_ID = t2.sql_id inner join v$session_wait t3 on t2.sid = t3.sid 

where t3.wait_class <> 'Idle'

三、查询latch相关的统计数据

select latch#, name, gets, misses, sleeps 

from v$latch where latch#=98 

order by sleeps; 

select latch#,name,gets,misses,sleeps 

from v$latch 

where name like 'cache buffer%';

参考资料：

模拟试验–latch free之cache buffers chains