Very high buffer gets
- From: "Nahata, Naveen (US - Glen Mills)" <nnahata@xxxxxxxxxxxx>
- To: <oracle-l@xxxxxxxxxxxxx>
- Date: Mon, 13 Feb 2006 11:03:43 -0500
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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