RE: Very high buffer gets

From: fairlie rego <fairlie_r_at_yahoo.com>
Date: Mon, 13 Feb 2006 16:22:22 -0800 (PST)
Message-ID: <20060214002222.2237.qmail@web31909.mail.mud.yahoo.com>

Hi Naveen    

  Isn't this the same issue as described in Metalink note 310923.1    

  Regards,
  Fairlie

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

      9.2.0.6 (on HP-UX)     

---

  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, t.flags, 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

in
('DEF$_AQCALL' , 'DEF$_AQERROR') and t.schema = 'SYSTEM')for update of 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
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Received on Mon Feb 13 2006 - 18:22:22 CST