Re: OCM Stats Module and DBA_TAB_MODIFICATIONS
- From: Kellyn Pedersen <kjped1313@xxxxxxxxx>
- To: Adam Musch <ahmusch@xxxxxxxxx>
- Date: Fri, 19 Feb 2010 11:42:10 -0800 (PST)
I should have clarified- I meant the ORACLE_OCM- start to finish, up to the
stats gathering step which you detailed below
I think you just gave me my answer though:
3. It gathers what it can until the window closes (6am local; 6AM Monday) .
I am hitting into the close of the window on this database, (and this is the
smallest of our databases!!), and it's making, (in my opinion) poor choices on
the stats collection, including sample sizes on our very large databases and is
never getting to a number of tables. My scripts that use
the DBA_TAB_MODIFICATIONS table as the source, are able to process through the
entire table since I specify more manageable sample sizes and better choices
for method options...
If you ever need anyone to "vent" to about the "evils" of this delivery, let me
know, I'm right there with you and have disabled it on all my primary
databases- returning to my own statistics gathering, but utilizing the handy
DBA_TAB_MODIFICATIONS table. I just knew I was missing a piece to the puzzle
as to why I have the table retention oddity!
Thanks!
Kellyn
--- On Fri, 2/19/10, Adam Musch <ahmusch@xxxxxxxxx> wrote:
From: Adam Musch <ahmusch@xxxxxxxxx>
Subject: Re: OCM Stats Module and DBA_TAB_MODIFICATIONS
To: "Kellyn Pedersen" <kjped1313@xxxxxxxxx>
Cc: "oracle Freelists" <oracle-l@xxxxxxxxxxxxx>
Date: Friday, February 19, 2010, 11:52 AM
From what I can tell, the OCM job isn't supposed to. In my systems,
MGMT_STATS_CONFIG_JOB invokes ORACLE_OCM.MGMT_CONFIG.collect_stats, which
submits this to run on each instance:
ORACLE_OCM.MGMT_DB_LL_METRICS.collect_stats_metrics('ORACLE_OCM_CONFIG_DIR')
That' puts out to a file in the directory the results of
MGMT_DB_LL_METRICS.collect_db_feature_usage;
MGMT_DB_LL_METRICS.collect_high_water_mark_stats;
MGMT_DB_LL_METRICS.collect_db_cpu_usage;
That job is all about monitoring what features have of the database have been
used, I suppose for automatic licensure compliance. It's not about gathering
database optimizer stats, that's SYS.GATHER_STATS_JOB.
That job is subject to the maintenance windows, and if you have too many
objects which need to have their statistics gathered -- after all, Oracle's
being helpful with their defaults of gathering histograms all over creation
according to undocumented logic -- you may have a problem where (defaults in
parentheses):
1. The stats job kicks off at the start of the maintenance window (10pm local
M-F; 12 AM Saturday)
2. The stats job iterates over each table in DBA_TAB_MODIFICATIONS, serially
-- one segment at a time -- gathering table, table partition, table
subpartition, index, index partition, and index subpartition statistics,
including histograms. This appears to be in schema-table order. As each table
finishes, it's removed from the DBA_TAB_MODIFICATIONS view.
3. It gathers what it can until the window closes (6am local; 6AM Monday) .
The next window, it starts over. However, any tables which didn't get to in
the previous run didn't get gathered, and if there's enough activity (approx
10% of table volume inserted/updated/deleted and/or table truncated), a table
which got gathered yesterday will jump ahead of tables which got skipped in the
previous run.
You may wish to issue a call to DBMS_STATS.FLUSH_DATABASE_MONITORING and get a
snapshot of what those views looked like at the start and end of the
maintenance window to see what tables getting fresher statistics. Also, the
LAST_ANALYZED column in DBA_(TABLES | INDEXES | TAB_PARTITIONS |
TAB_SUBPARTITIONS | IND_PARTITIONS | IND_SUBPARTITIONS) might be useful to see
what got touched. If the job isn't finishing, you should get rows in
DBA_SCHEDULER_JOB_LOG where JOB_NAME = 'GATHER_STATS_JOB' and STATUS =
'STOPPED'.
The process Oracle delivered here, in short, is awful. It works fine for small
systems, but for larger systems including partitioning and subpartitioning, the
histogram defaults require reiterative scanning of partitions and tables as it
doesn't do any rational digesting. It's riduclously underdocumented, to boot.
I could go off on how evil histogram defaults are, especially in a world where
you can't tell Oracle about column correlation and how badly that skews
cardinality estimates, but that's for another day.
On Fri, Feb 19, 2010 at 11:33 AM, Kellyn Pedersen <kjped1313@xxxxxxxxx> wrote:
Good thought, but nope, not in this environment, no stats are locked in this
OLTP database...
I have this problem in all my databases and for the other databases that I
manage exclusively, I've moved to my own scripts that utilize this table as
it's source, but I really want to know what's up and why the OCM auto stats job
doesn't clear this table out..:(
The other issue I have, is that the job that inserts into DBA_TAB_MODIFICATIONS
appears to scan different areas of the database in certain intervals, so it can
suddenly populate up to 14,000 rows to gather stats on when there are many
partitions involved, (I would think that a partition level trigger to populate
would be more efficient...)
Due to this, I have my script looking at the row count by date and if it
reaches a threshold, it will exit and keep that days updates for the weekend
run to not impact production. To me, that just seems like a bug- to not view
the staleness at a partition level vs. table level...
Kellyn
--- On Fri, 2/19/10, Adam Musch <ahmusch@xxxxxxxxx> wrote:
From: Adam Musch <ahmusch@xxxxxxxxx>
Subject: Re: OCM Stats Module and DBA_TAB_MODIFICATIONS
To: kjped1313@xxxxxxxxx
Cc: "oracle Freelists" <oracle-l@xxxxxxxxxxxxx>
Date: Friday, February 19, 2010, 9:55 AM
Are the stats on those tables locked (STATTYPE_LOCKED in DBA_TAB_STATISTICS)?
On Fri, Feb 19, 2010 at 10:47 AM, Kellyn Pedersen <kjped1313@xxxxxxxxx> wrote:
Here's my stupid question for the week- I'm sure I'm just missing something
here...
Oracle 10.2.0.4 on Linux- I'm under the impression from all that I've read,
that the DBA_TAB_MODIFICATIONS table contains tables, along with row count
changes for tables that are considered "stale" and need to have stats gathered.
Should the OCM auto job for gathering stats, that's driven off the
DBA_TAB_MODIFICATIONS table, clear ALL objects that are in that table once it
runs on it's schedule?
I have tables back from the conception of the database in
DBA_TAB_MODIFICATIONS. No gathering of stats have been taken by the auto stats
job. This is both for the SYS and the application schema, so it's not just at
a level where I would look for a setting that says, "don't gather stats on
system schema objects".
I've verified that if I gather stats on a table that is in
DBA_TAB_MODIFICATIONS, it updates and removes the entry from the table, so it's
not an issue of not updating the table and the stats job doing it's part- it
simply is disregarding certain tables in the DBA_TAB_MODIFICATIONS table. The
jobs are scheduled precisely as they should and are running to completion, no
issues there.
There isn't a lot of documentation on the OCM stats module, but maybe I'm not
searching for the right terms...
Can anyone tell me- is there a bug in 10g for this feature? Is this table
utilized differently than what I think it is? Is Kellyn just not searching with
the right terminology? :)
Thanks!
Kellyn Pedersen
Multi-Platform DBA
I-Behavior Inc.
http://www.linkedin.com/in/kellynpedersen
www.dbakevlar.blogspot.com
"Go away before I replace you with a very small and efficient shell script..."
--
Adam Musch
ahmusch@xxxxxxxxx
--
Adam Musch
ahmusch@xxxxxxxxx
Other related posts:
