RE: Very high buffer gets

From: Nahata, Naveen (US - Glen Mills) <nnahata_at_deloitte.com>
Date: Tue, 14 Feb 2006 08:34:49 -0500
Message-Id: <7498054865834541A868EA11FDD4D078013CE98C@usndc0410.us.deloitte.com>

Yes, I have already seen this note. Ultimately, I will go ahead with what is recommended in the metalink note, but I fail to understand why too many blocks are being read and hence posted the query in this forum. Probably its because of my lack of experience with AQ/IOTs I cannot come up with a reasonable explanation.  

Naveen

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From: fairlie rego [mailto:fairlie_r_at_yahoo.com] Sent: Monday, February 13, 2006 7:22 PM
To: Nahata, Naveen (US - Glen Mills)
Cc: oracle-l_at_freelists.org; Alex Gorbachev Subject: RE: Very high buffer gets

Hi Naveen  

Isn't this the same issue as described in Metalink note 310923.1
<https://www.metalink.oracle.com/metalink/plsql/f?p=130:14:4200270662525
378742::::p14_database_id,p14_docid,p14_show_header,p14_show_help,p14_bl ack_frame,p14_font:NOT,310923.1,1,1,1,helvetica>  

Regards,
Fairlie

"Nahata, Naveen (US - Glen Mills)" <nnahata_at_deloitte.com> wrote:

        9.2.0.6 (on HP-UX)

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	From: Alex Gorbachev [mailto:gorbyx_at_gmail.com] 
	Sent: Monday, February 13, 2006 4:07 PM
	To: Nahata, Naveen (US - Glen Mills)
	Cc: oracle-l_at_freelists.org
	Subject: Re: Very high buffer gets
	
	
	If I recall correctly this is a bug in AQ handling. I think you

should be able to find it easilly on Metalink. I think Oracle attributes LIO that is caused by QMN during queue maintenance (delayed purging) instead of attributing those LIOs to the statements causing it. What is your version?                  

        2006/2/13, Nahata, Naveen (US - Glen Mills)
<nnahata_at_deloitte.com>:

                Hi All,                 

                After tracing the QMON process which was doing lots of buffer gets, I

                found this culprit. None of the three tables involved in the query have

                more than 2 blocks. I know the nested loop is iterating over the blocks

                but that should still not account for more than 200 blocks / execution.

                Why do I see such high values for buffer gets?                 

                select t.schema , t.name <http://t.name/> , t.flags, q.name <http://q.name/>

		from
		system.aq$_queue_tables t,
sys.aq$_queue_table_affinities aft,
		system.aq$_queues q where aft.table_objno = t.objno and
		aft.owner_instance =
		  :1 and        q.table_objno = t.objno and q.usage = 0
and
		bitand(t.flags, 4+16+32+64+128+256) = 0       and NOT
( t.name <http://t.name/> 
		in
		('DEF$_AQCALL' , 'DEF$_AQERROR') and t.schema =
'SYSTEM')for update of 
		t.name <http://t.name/> , aft.table_objno skip locked
		
		
		call     count       cpu    elapsed       disk
query    current
		rows
		------- ------  -------- ---------- ----------
---------- ---------- 
		----------
		Parse       84      0.00       0.03          0
0          0
		0
		Execute     84      0.03       0.03          0
0          0
		0
		Fetch       84     11.87      47.87        266
536288      45281 
		4627
		------- ------  -------- ---------- ----------
---------- ----------
		----------
		total      252     11.90      47.94        266
536288      45281
		4627
		
		Misses in library cache during parse: 1 
		Optimizer goal: CHOOSE
		Parsing user id: SYS   (recursive depth: 1)
		
		Rows     Row Source Operation
		-------
---------------------------------------------------
		    60  FOR UPDATE
		    62   HASH JOIN
		    59    NESTED LOOPS
		    59     TABLE ACCESS FULL AQ$_QUEUE_TABLES
		    59     TABLE ACCESS BY INDEX ROWID
AQ$_QUEUE_TABLE_AFFINITIES
		    59      INDEX UNIQUE SCAN AQ$_QTABLE_AFFINITIES_PK
(object id 2400)
		    64    TABLE ACCESS FULL AQ$_QUEUES 
		
		Naveen
		
		
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	-- 
	Best regards,
	Alex Gorbachev 

Fairlie Rego
Senior Oracle Consultant
Optus Telecommunications
www.optus.com.au <http://www.optus.com.au/> Mobile: +61 4 02 792 405
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Received on Tue Feb 14 2006 - 07:34:49 CST