Latch: cache buffer chains等待事件的学习

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、查询各等待时间的含义：

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：