概述
今天主要介绍Oracle数据库在并行操作过程中 slave 进程和 QC 进程经常遇到的等待事件及常用脚本。
一、PX Deq: Execution Msg，PX Deq: Execute Reply等待事件
1、PX Deq: Execution Msg
Occurs when a parallel slave is waiting to be told what to do. This is normally considered an idle event, but can cause excessive CPU in some cases.
该事件是并行查询中的常见事件。当PQ slave进程在等待QC告诉它要做什么的时候就会出现此事件(eg: when waiting to be told parse / execute / fetch etc..)
v$session_wait 中该等待事件对应的参数：
P1 = sleeptime/senderid
P2 = passes
P3 = not used
我们可以使用如下语句获取转换sleeptime/senderid的相关信息：
set SERVEROUTPUT on
undef p1
declare
inst varchar(20);
sender varchar(20);
begin
select bitand(&&p1, 16711680) - 65535 as SNDRINST,
decode(bitand(&&p1, 65535),65535, 'QC', 'P'||to_char(bitand(&&p1, 65535),'fm000') ) as SNDR
into inst , sender
from dual
where bitand(&&p1, 268435456) = 268435456;
dbms_output.put_line('Instance = '||inst);
dbms_output.put_line('Sender = '||sender );
end;
/ 如果P1的值为空，则意味slave 不需要等待任何进程
比如p1的值为268501004，则上面的sql会返回：
Instance = 1
Sender = P012
passes 进程在得到信息之前循环轮转等待的次数
该等待事件是一个空闲等待事件，当此等待事件出现,进程会持续等待并逐渐增加等待次数直到获取信息！
解决方法：
作为 Coordinator 的 Process 在获取 Slave 进程的数据时，反应太慢了，导致某些 Slave进行因为 Queue 满而不得不等待，进而拖慢了整个并行执行的速度。
这常常是由于 CPU 数目不足或者 系统中运行的 进程太多导致。可考虑 减小并行度。
2、PX Deq: Execute Reply
Occurs when the query coordinator is waiting for a response from a parallel slave. This is normally considered an idle event, but can cause excessive CPU in some cases.
Waiting Process: QC
协调器正在等待一个 从slaves 进程对控制信息的响应(确认通知)或者期望从slave进程集中获取数据。这个等待事件意味着QC等待slaves结束执行sql 并且将结果集发送给QC
v$session_wait 中该等待事件对应的参数：
P1 = sleeptime/senderid
P2 = passes
P3 = not used
我们可以使用如下语句获取转换sleeptime/senderid的相关信息：
set SERVEROUTPUT on
undef p1
declare
inst varchar(20);
sender varchar(20);
begin
select bitand(&&p1, 16711680) - 65535 as SNDRINST,
decode(bitand(&&p1, 65535),65535, 'QC', 'P'||to_char(bitand(&&p1, 65535),'fm000') ) as SNDR
into inst , sender
from dual
where bitand(&&p1, 268435456) = 268435456;
dbms_output.put_line('Instance = '||inst);
dbms_output.put_line('Sender = '||sender );
end;
/ 如果P1的值为空，则意味slave 不需要等待任何进程
比如p1的值为268501004，则上面的sql会返回：
Instance = 1
Sender = P012
等待时间：
这是非空闲等待时间，QC 等待从slave 的响应或者查询的数据结果
解决办法：
非优化的sql语句肯能是导致此等待事件的原因：slaves 需要花费很长时间来执行sql 语句而qc又在等待slave返回数据。
优化sql，查看slave 在执行的语句以及其执行计划，并做出尽量的优化,以便减少slave执行sql语句的时间！
二、相关脚本
1、gives an overview of all running parallel queries with all slaves.It shows the if a slave is waiting and for what event it waits.
select decode(px.qcinst_id, NULL, username, ' - ' || lower(substr(pp.SERVER_NAME, length(pp.SERVER_NAME) - 4, 4))) "Username", decode(px.qcinst_id, NULL, 'QC', '(Slave)') "QC/Slave", to_char(px.server_set) "SlaveSet", to_char(s.sid) "SID", to_char(px.inst_id) "Slave INST", decode(sw.state, 'WAITING', 'WAIT', 'NOT WAIT') as STATE, case sw.state WHEN 'WAITING' THEN substr(sw.event, 1, 30) ELSE NULL end as wait_event, decode(px.qcinst_id, NULL, to_char(s.sid), px.qcsid) "QC SID", to_char(px.qcinst_id) "QC INST", px.req_degree "Req. DOP", px.degree "Actual DOP" from gv$px_session px, gv$session s, gv$px_process pp, gv$session_wait sw where px.sid = s.sid(+) and px.serial# = s.serial#(+) and px.inst_id = s.inst_id(+) and px.sid = pp.sid(+) and px.serial# = pp.serial#(+) and sw.sid = s.sid and sw.inst_id = s.inst_id order by decode(px.QCINST_ID, NULL, px.INST_ID, px.QCINST_ID), px.QCSID, decode(px.SERVER_GROUP, NULL, 0, px.SERVER_GROUP), px.SERVER_SET, px.INST_ID /
2、shows for the PX Deq events the processes that are exchange data.
select sw.SID as RCVSID, decode(pp.server_name, NULL, 'A QC', pp.server_name) as RCVR, sw.inst_id as RCVRINST, case sw.state WHEN 'WAITING' THEN substr(sw.event, 1, 30) ELSE NULL end as wait_event, decode(bitand(p1, 65535), 65535, 'QC', 'P' || to_char(bitand(p1, 65535), 'fm000')) as SNDR, bitand(p1, 16711680) - 65535 as SNDRINST, decode(bitand(p1, 65535), 65535, ps.qcsid, (select sid from gv$px_process where server_name = 'P' || to_char(bitand(sw.p1, 65535), 'fm000') and inst_id = bitand(sw.p1, 16711680) - 65535)) as SNDRSID, decode(sw.state, 'WAITING', 'WAIT', 'NOT WAIT') as STATE from gv$session_wait sw, gv$px_process pp, gv$px_session ps where sw.sid = pp.sid(+) and sw.inst_id = pp.inst_id(+) and sw.sid = ps.sid(+) and sw.inst_id = ps.inst_id(+) and p1text = 'sleeptime/senderid' and bitand(p1, 268435456) = 268435456 order by decode(ps.QCINST_ID, NULL, ps.INST_ID, ps.QCINST_ID), ps.QCSID, decode(ps.SERVER_GROUP, NULL, 0, ps.SERVER_GROUP), ps.SERVER_SET, ps.INST_ID
3、shows for long running processes what are the slaves do.
select decode(px.qcinst_id, NULL, username, ' - ' || lower(substr(pp.SERVER_NAME, length(pp.SERVER_NAME) - 4, 4))) "Username", decode(px.qcinst_id, NULL, 'QC', '(Slave)') "QC/Slave", to_char(px.server_set) "SlaveSet", to_char(px.inst_id) "Slave INST", substr(opname, 1, 30) operation_name, substr(target, 1, 30) target, sofar, totalwork, units, start_time, timestamp, decode(px.qcinst_id, NULL, to_char(s.sid), px.qcsid) "QC SID", to_char(px.qcinst_id) "QC INST" from gv$px_session px, gv$px_process pp, gv$session_longops s where px.sid = s.sid and px.serial# = s.serial# and px.inst_id = s.inst_id and px.sid = pp.sid(+) and px.serial# = pp.serial#(+) order by decode(px.QCINST_ID, NULL, px.INST_ID, px.QCINST_ID), px.QCSID, decode(px.SERVER_GROUP, NULL, 0, px.SERVER_GROUP), px.SERVER_SET, px.INST_ID
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