Very high buffer gets

From: Nahata, Naveen (US - Glen Mills) <nnahata_at_deloitte.com>
Date: Mon, 13 Feb 2006 11:03:43 -0500
Message-Id: <7498054865834541A868EA11FDD4D078013CE322@usndc0410.us.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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http://www.freelists.org/webpage/oracle-l Received on Mon Feb 13 2006 - 10:03:43 CST