RE: Missing SQL in DBA_HIST_SQLSTAT
Date: Tue, 2 Nov 2010 13:53:04 +0800
Message-ID: <E3CCEA4C11521E43A343441089E9E55501D40530_at_ESNG17P32003B.csfb.cs-group.com>
Dave,
I've noticed that the quality of AWR data collection varies a lot depending on the application profile.
I am not an expert on database internals ("Tanel!!") but this is what I have worked out; my problems in this area are due to flushing of the sql area. As you can see the metrics in gv$sqlstat; then it may not be relevant.
A database hosting an application that executes most of its workload during a batch window seems to have lower quality of AWR data collection during its busiest period. One major reason for this is aging of single-execution statements. The SQL Area sizes automatically based on the workload of the entire day, so when a large burst of statements is run (= overnight batch cycle), the SQL area will not resize up immediately (if at all), and single-execution statements will be aged out first before the next AWR snap occurs. I've seen this happen a lot - the statement metrics are no longer visible in gv$sqlstat at snap-time so it never goes into AWR.
Aging of metrics for statements is only a problem if the database needs to flush the sql area: it can be flushed by other activity. Single-execution statements appear to be chosen first, irrespective of how long they ran for - AFAIK no attempt is made to classify which single-execution statements may still have important metrics.
A flush offender is a stats gather on objects with a lot of column statistics: it generates a large number of recursive statements for each column (select min(), max() etc) in a short span, which can flush a lot of metrics from the sql area. The AWR partition rolling job can also generate a lot of statements - this generally runs once a day.
I've raised an SR to find out if there is any way to keep statements in the SQL area until they have been snapped into AWR, but support told me to go away, its supposed to be like that. It is not possible to directly set the size of the sql area to improve metric retention. Statement pinning doesn't work either: the text is kept, but the metrics (gv$sqlstat) are flushed! I hit a brick wall on this. It seems a design flaw to me - the sql area flush process should assess the metrics for each statement to see if it is eligible for AWR retention. Not trivial I know, but a lot depends on AWR quality.
Interesting that you have seen this on a 4-node RAC - the worst offender I have is a 3-node RAC - is there any chance AWR is not picking up metrics across all instances?
HTH
Mark Teehan
Singapore
-----Original Message-----
From: oracle-l-bounce_at_freelists.org [mailto:oracle-l-bounce_at_freelists.org] On Behalf Of Herring Dave - dherri
Sent: 26 October 2010 06:08
To: ORACLE-L
Subject: Missing SQL in DBA_HIST_SQLSTAT
Has anyone checked into the accuracy and reliability of AWR SQL stats? From checks I've done it appears that a lot of SQL statements are not getting captured in AWR snapshots.
Here's our config: 4-node RAC, Oracle 10.2.0.2, RH 4.x, 30-minute AWR snapshots.
We've left TOPNSQL as "DEFAULT", which in our case should be enough:
SELECT topnsql
FROM sys.wrm$_wr_control;
TOPNSQL
2000000000
This means that the first 2 BILLION SQL statements per category will be selected (seems rather excessive!). Yet I ran a few checks where I compared (GV$SQL.LAST_ACTIVE_TIME - snapshot interval) to the MAX(end_interval_time) for snapshots (all by SQL_ID) and found over 1600 statements that are executed yet no updates are captured by AWR.
As a double check, I reviewed data in DBA_HIST_ACTIVE_SESS_HISTORY and found that 1 of the 1600 statements from above is executed multiple times per day, yet the last DBA_HIST_SQLSTAT snapshot it's in is from 4 months ago.
Has anyone else seen this type of thing and/or done detailed checking on the accuracy of AWR SQL data?
BTW, I tried tracing the snapshot activity and can't get past what's done to populate internal objects X$KEWRSQLIDTAB and X$KEWRSQLCRIT.
Dave Herring | DBA
Acxiom Global Technology Solutions
630-944-4762 office | 630-430-5988 cell | 630-944-4989 fax 1501 Opus Pl | Downers Grove, IL, 60515 | U.S.A. | www.acxiom.com Service Desk: 888-243-4566, https://servicedesk.acxiom.com, GSCA_at_DNB.com
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Received on Tue Nov 02 2010 - 00:53:04 CDT