RE: Very high buffer gets

From: Mark W. Farnham <mwf_at_rsiz.com>
Date: Tue, 14 Feb 2006 08:09:10 -0500
Message-ID: <KNEIIDHFLNJDHOOCFCDKIELEHKAA.mwf@rsiz.com>

How did the original poster determine that the tables have only 2 blocks or less each? A less than 100 percent statistics computation, perchance?

Perhaps

select count(non-indexed-column) from aq$_<tab>

(without any joins or other complications) would show more blocks below the high water mark than you think you have.

Regards,

mwf
  -----Original Message-----
  From: oracle-l-bounce_at_freelists.org
[mailto:oracle-l-bounce_at_freelists.org]On Behalf Of Alex Gorbachev   Sent: Monday, February 13, 2006 4:07 PM   To: nnahata_at_deloitte.com
  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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Received on Tue Feb 14 2006 - 07:09:10 CST