RE: Killer SQL and PGA
- From: "Mark W. Farnham" <mwf@xxxxxxxx>
- To: <oracle-l@xxxxxxxxxxxxx>
- Date: Tue, 31 Jan 2012 09:17:18 -0500
ulimit?
The tradeoff here is whether to utilize virtual memory to allow gargantuan
jobs to theoretically be processed at all. Seymour Cray nailed that in one
when he was asked why his systems did not support virtual memory.
It will always be possible to concoct queries and problems that cannot be
solved within the limits of any arbitrary amount of real memory.
And of course if you put in ulimit limitations and exceed them, then within
some timeslice or interrupt that allows the kernel to figure that out the
violating process gets killed. The side effects of the operating system
killing Oracle processes is too complex to analyze in the general case.
You mentioned that you had recently moved from 9i to 11gR2, but it is not
clear to me whether your "analyst" had previously run the same query in 9i
successfully. If so, *probably* it used an entirely different plan. Then
again, I've often seen folks test limits on an upgraded system, for example
in this context, by supplying a longer "in" list until something breaks.
Those are the folks you hope to identify to play games on a "conference room
pilot" or quality assurance test upgrade before you go live.
Likewise, proper education to help them avoid unleashing experimental loads
on the production system is usually part of the solution.
Regards,
mwf
-----Original Message-----
From: oracle-l-bounce@xxxxxxxxxxxxx [mailto:oracle-l-bounce@xxxxxxxxxxxxx]
On Behalf Of Robert Laverty
Sent: Monday, January 30, 2012 6:13 PM
To: oracle-l@xxxxxxxxxxxxx
Subject: Killer SQL and PGA
We had a problem with our PGA growing larger than physical memory, dragging
the system down until we were forced to restart the database. We recently
upgraded from 9i to 11gR2 on Solaris with 16Gb physical memory hosting an
OLTP application. 4Gb is used for SGA and 400Mb for PGA_aggregate_target.
AMM and ASMM have not been enabled. Workarea_size_policy is set to AUTO.
This is a simple database. No RAC, no shared servers, no parallel
processing.
One of our analysts launched what appeared to be a simple ad hoc query:
select * from deniedhist where icn in ('1', '2', '3', ... , 'x').
Unfortunately, the analyst had over 400 ICNs listed. DENIEDHIST is a UNION
ALL view representing a faux-partitioned array of 129 tables, each with its
own unique index on ICN. The execution plan (my comments included) shows
that it is searching each underlying table 'x' number of times, using the
unique index each time. This is the plan from a test version of the SQL
with only 5 ICNs. The rest of the plan shows the iteration through the
other 128 underlying tables (other views of the plan show the unique table
and index names).
1422 SELECT STATEMENT REMOTE CHOOSE
1421 VIEW POS.DENIEDHIST
1420 UNION-ALL
11 CONCATENATION
-- This is table 1 of 129 in the view
2 TABLE
ACCESS BY INDEX ROWID POS.DENIED_HISTORY
1 INDEX UNIQUE SCAN UNIQUE POS.DENIED_HISTORY_ICN
4 TABLE
ACCESS BY INDEX ROWID POS.DENIED_HISTORY
3 INDEX UNIQUE SCAN UNIQUE POS.DENIED_HISTORY_ICN
6 TABLE
ACCESS BY INDEX ROWID POS.DENIED_HISTORY
5 INDEX UNIQUE SCAN UNIQUE POS.DENIED_HISTORY_ICN
8 TABLE
ACCESS BY INDEX ROWID POS.DENIED_HISTORY
7 INDEX UNIQUE SCAN UNIQUE POS.DENIED_HISTORY_ICN
10 TABLE
ACCESS BY INDEX ROWID POS.DENIED_HISTORY
9 INDEX UNIQUE SCAN UNIQUE POS.DENIED_HISTORY_ICN
22 CONCATENATION
-- This is table 2 of 129 in the view
...
I estimated that the original version probably had over 120,000 steps in its
execution plan. It ran for nearly an hour until memory reached a critical
point, almost 20 Gb and paging like mad, that required us to abort the
database and restart. I can run the full original query on a standby
machine which is physically identical but is still running 9i. It finishes
in 5 minutes and the PGA only grows to 500 Mb. The execution plans, at
least for the small test version of the query, are the same on both
machines.
I understand the problems with large 'IN' lists. There was a conversation
on that topic here at the end of October . Educating the analyst and
redesigning the query are secondary concerns. I also realize, after reading
a bunch of Tom Kyte posts, that pga_aggregate_target will not constrain the
growth of the PGA.
My real question is why the 11g memory management, without AMM or ASMM,
would allow the PGA to grow so large. In 15 years of operations, there must
have been similar bad queries against the database. This happened a day
after the 11g upgrade. Any suggestions?
Bob Laverty
Molina Medicaid Solutions
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