ASSM wreck

Yesterday I introduced a little framework I use to avoid the traps inherent in writing PL/SQL loops when modelling a session that does lots of simple calls to the database. I decided to publish the framework because I had recently come across an example where a series of SQL statements gives a very different result from a single PL/SQL block.

 

Repetition

One of the problems of building models of Oracle activity is that it’s easy to build the wrong model. One of the commonest issues appears with repetitive actions – how do you write code that repeats a simple action many times in a row. It’s often enough to write a simple pl/sql loop but there are cases where a pl/sql loop behaves very differently from a long list of individual SQL statements – which is why I’ve occasionally used a very simple-minded approach to avoid that particular trap.

 

Ambiguity

“There is no space problem.”

If you saw this comment in the middle of a thread about some vaguely described Oracle problem, which of the following would you think was the intended meaning:

There is a problem – we have no space.

We do not have a problem with space

Wouldn’t it make life so much easier to choose between:

We are not seeing any Oracle errors.

We are seeing Oracle error: “ORA-01653: unable to extend table X by N in tablespace Z”

(That’s just one of many possible space-related errors, of course.)

Small Tables

Here’s a note I’ve been meaning to research and write up for more than 18 months – ever since Dion Cho pinged a note I’d written about the effects of partitioning because of a comment it made about the “2% small table threshold”.

It has long been an item of common knowledge that Oracle has a “small table threshold” that allows for special treatment of data segments that are smaller than two percent of the size of the buffer cache, viz:

Visitors etc.

Well, I’ve finally reached another “landmark” number in the page views – 2,000,000 views (after four and a half years) – so it’s time for a little statistical summary for comparison with the last landmark.

Looking at just the results for the last 12 months, here are some numbers in different categories.

Upgrade Whoa

(The title’s a pun, by the way – an English form of humour that is not considered good unless it’s really bad.)

Very few people try to email me or call me with private problems – which is the way it should be, and I am grateful to my audience for realizing that this blog isn’t trying to compete with AskTom – but I do get the occasional communication and sometimes it’s an interesting oddity that’s worth a little time.

Today’s blog item is one such oddity – it was a surprise, it looked like a nasty change in behaviour, and it came complete with a description of environment, and a neatly formatted, complete, demonstration. For a discussion of the problem in Spanish you can visit the blog of John Ospino Rivas, who sent me the original email and has written his own blog post on the problem.

ASSM ouch!

Here’s a nasty little surprise I got last week while investigating an oddity with stats collection. I wanted to create a table in an ASSM tablespace and populate it from two or three separate sessions simultaneously so that I could get some “sparseness” in the data load. So I created a table and ran up 17 concurrent sessions to insert a few rows each. Because I wanted to know where the rows were going I got every session to dump the bitmap space management block at the start of the segment – the results were surprising.

buffer flush

This is part 2 of an article on the KEEP cache. If you haven’t got here from part one you should read that first for an explanation of the STATE and CUR columns of the output.

Here’s a little code to demonstrate some of the problems with setting a KEEP cache – I’ve set up a 16MB cache, which gives me 1,996 buffers of 8KB in 10.2.0.3, and then created a table that doesn’t quite fill that cache. The table is 1,900 data blocks plus one block for the segment header (I’ve used freelist management to make the test as predictable as possible, and fixed the pctfree to get one row per block).

Buffer States

Here’s a bit of geek stuff that I’ve been meaning to write up for nearly a year – to the day, more or less – and I’ve finally been prompted to finish the job off by the re-appearance on the OTN database forum of the standard “keep cache” question:

Why isn’t Oracle keeping an object “properly” when it’s smaller than the db_keep_cache_size and it has been assigned to the buffer_pool keep ?

This is a two-part note – and in the first part I’m just going to run a query and talk about the results. The query is one that has to be run by SYS because it references a couple of x$ structures, and this particular version of the query was engineered specifically for a particular client.

Show stuff

There are various “show” commands in SQL*Plus that are convenient short-cuts for simple SQL commands, for example:

show parameter 
show recyclebin
show sga
show spparameter

Index Rebuilds

I wrote a short note last week that linked to a thread on the Russian Oracle forum about indexing, and if you’ve followed the thread you will have seen a demonstration that seemed to be proving the point that there were cases where an index rebuild would be beneficial.

Of course it’s not difficult to come up with cases where index rebuilds should make a difference – but it’s harder to come up with demonstrations that look realistic, so I thought I’d review the example to explain why it doesn’t really work as a good example of why you might need to think about rebuilding some production index.

Statspack Reports

A couple of weeks ago I listed a number of scripts from 11.2.0.2 relating to AWR reports – it seems only sensible to publish a corresponding list for Statspack. In fact, there are two such lists – one for “traditional” Statspack, and one for “standby statspack” – a version of statspack you can pre-install so that you can run statspack reports against a standby database.

Statspack – the basic source of information is $ORACLE_HOME/rdbms/admin/spdoc.txt

spreport.sql    ->  Generates a Statspack Instance report
sprepins.sql    ->  Generates a Statspack Instance report for the database and instance specified
sprepsql.sql    ->  Generates a Statspack SQL report for the SQL Hash Value specified
sprsqins.sql    ->  Generates a Statspack SQL report for the SQL Hash Value specified, for the database and instance specified

sppurge.sql     ->  Purges a limited range of Snapshot Id's for a given database instance
sptrunc.sql     ->  Truncates all Performance data in Statspack tables
spuexp.par      ->  An export parameter file supplied for exporting the whole PERFSTAT user

Standby Statspack – the basic source of information is $ORACLE_HOME/rdbms/admin/sbdoc.txt

sbreport.sql      - Create a report

sbaddins.sql      - Add a standby database instance to the configuration
sblisins.sql      - List instances in the standby configuration
sbdelins.sql      - Delete an instance from the standby configuration

sbpurge.sql       - Purge a set of snapshots

A warning note about standby statspack – there is a significant structural change from 11.2.0.1 to 11.2.0.2 allowing support of multiple standby databases. I haven’t looked closely at it yet, but it did cross my mind that it might be possible to install the 11.2.0.2 version in an 11.2.0.1 database to avoid late upgrade issues – I may get around to testing the idea one day.

Valid Values

How do you find out if there are any restrictions (or interesting, possibly undocumented, values) for the system parameters ? Once upon a time I used to use a value that was (probably) illegal so that I could find out from the resulting error message what the list of valid values was, for example:

MOS

or the Oracle support interface formerly known as Metalink.

In case you haven’t come across it, or if you’ve lost the URL, there is an HTML interface which, on the downside, doesn’t offer all the features but, on the upside, does seem to be a little more robust and a little faster. (I’ve added links to both the flash and html versions in my blogroll.)

Free ASH

If you’re running a version of Oracle older than 10g (where v$active_session_history appeared), or if you’re not using Enterprise Edition, or if you just don’t want to pay for the Diagnostic Pack licence, here are some links relating a free Java program that emulates the “Top Sessions” output of the Enterprise Manager screen:

http://timurakhmadeev.wordpress.com/2010/02/18/ash-viewer/
http://sourceforge.net/projects/ashv/

Important Note:

If you want to query v$active_session_history (or any of the AWR objects) then you need to purchase the licence for the Diagnostic Pack). If you aren’t licensed you should only use the program in emulation mode.