查詢gv$session引發大量的PX Deq Credit: send blkd事件

一個使用者提交了一個awr報告，發現有大量的PX Deq Credit: send blkd事件:

Top 5 Timed Events

Event	Waits	Time(s)	Avg Wait(ms)	% Total Call Time	Wait Class
CPU time		3,023		43.0
db file sequential read	320,800	1,205	4	17.1	User I/O
PX Deq Credit: send blkd	452	998	2,207	14.2	Other
library cache lock	19,034	425	22	6.0	Concurrency
latch: library cache	3,023	366	121	5.2	Concurrency

Oracle對這個事件的描述：

This wait event indicates that we are doing a non-blocking test to see if any channel has returned a message. We should see that although there is a high number of these events, the time accumulated to it should be zero (or really low).

這個事件按Oracle的說法為空閒事件，不用太去關注，多發生在並行DML時。但在客戶的系統中，並沒有使PDML;在TOP SQL中，找到有對gv$session的查詢語句，一條sql執行了872秒：

Elapsed Time (s)	CPU Time (s)	Executions	Elap per Exec (s)	% Total DB Time	SQL Id	SQL Module	SQL Text
872	0	1	871.68	12.40		PL/SQL Developer	select * from gv$session

在metalink上找到了多篇gv$session相關問題的文章;看來以後執行gv$session要慎重呀…

[@more@]