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CUR Gets

Here’s an interesting anomaly from the OTN database forum – PL/SQL track. It’s a single row delete from a table that does a huge number of db block gets and (in the first example shown) physical reads. Here’s the supplied output with autotrace enabled:

Debugger Dangers – Part 2

About 5 years ago I wrote about the risks that connecting to Oracle processes via debuggers may cause and what are (in my opinion) the safer and less safer options for taking stack samples from running Oracle processes.

In the end of that article I listed different options for getting a stack traces and whether they were safe or not.

For example, ORADEBUG-based process stack traces (DUMP ERRORSTACK, SHORT_STACK and event the process/system state dumps (at level 256 or higher) are not 100% safe – because they alter the execution path of the process they attached to. Your process may crash or get some error if you hit a bug (of course once you patch/fix the bug, you’ll be fine again – until you may hit the next bug).

An example bug is this:

List Exadata Storage Cell disk summary with cellpd.sql and cellpdx.sql scripts

In the previous post I explained how to list Exadata disk layout and topology details with the exadisktopo scripts, in this post I’ll introduce one celldisk overview script, which I use to quickly see the celldisk configuration, specs and error statuses. The cellpd.sql script (Cell Physical Disk) will show the following output:

List Exadata Disk Layout and Topology with the exadisktopo scripts

Here are two more Exadata scripts for listing the end-to-end ASM<->Exadata disk topology from V$ASM_ views and from V$CELL_CONFIG. These scripts see both the ASM level layout and the storage cell-level disk topology.

The exadisktopo.sql script shows all disks starting from the ASM diskgroup layer, going deeper and deeper all the way to the OS disk device level in the storage cells. It uses outer joins, so will show celldisks even if there are no corresponding grid disks allocated on them (or if there are no ASM disks using them). It also shows the Flash cards used as flash cache, thus there are no ASM disks on them usually.

V$CELL_THREAD_HISTORY – “ASH” for Exadata Storage Cells

Did you know that there’s something like Active Session History also in the Exadata storage cells? ;-)

The V$CELL_THREAD_HISTORY view is somewhat like V$ACTIVE_SESSION_HISTORY, but it’s measuring thread activity in the Exadata Storage Cells:

Speaking at E4 2013! … and some Exadata Patents good stuff

I’ll be co-speaking with Tyler Muth at E4  and we will be talking about Performance Visualization… some of you may already know that I’m really a big fan of Tableau and just to give you an example on what the tool can do I’ve consolidated all the different viz that I’ve done during my sizing & performance gigs here http://goo.gl/xZHHY and Tyler has also been doing a lot of Exadata performance & benchmarking gigs lately and there were a couple of times where we collaborated on specific viz (him doing it on R) just to validate each other’s work. In short, we are passionate about this stuff. And we are going to be awesome :)

Dynamic Sampling – 2

I’ve written about dynamic sampling in the past, but here’s a little wrinkle that’s easy to miss. How do you get the optimizer to work out the correct cardinality for a query like (the table creation statement follows the query):

select	count(*)
from	t1
where	n1 = n2
;

create table t1
as
with generator as (
	select	--+ materialize
		rownum id
	from dual
	connect by
		level <= 1e4
)
select
	mod(rownum, 1000)	n1,
	mod(rownum, 1000)	n2
from
	generator	v1,
	generator	v2
where
	rownum <= 1e6 ; 

If you’re running 11g and can changed the code there are a couple of easy options – adding a virtual column, or applying extended stats and then modifying the SQL accordingly would be appropriate.

gather_plan_statistics – 2

Some time ago – actually a few years ago – I wrote a note about the hint /*+ gather_plan_statistics */ making some informal comments about the implementation and relevant hidden parameters. I’ve recently discovered a couple of notes from Alexander Anokhin describing the feature in far more detail and describing some of the misleading side effects of the implementaiton. There are two parts (so far): part 1 and part 2.

 

Oracle Exadata Database Machine: Proving 160 Xeon E7 Cores Are As “Slow” As 128 Xeon E5 Cores?

Reading Data Sheets
If you are in a position of influence affecting technology adoption in your enterprise you likely spend a lot of time reading data sheets from vendors.  This is just a quick blog entry about something I simply haven’t taken the time to cover even though the topic at hand has always be a “problem.” Well, at least since the release of the Oracle Exadata Database Machine X2-8.

Oracle Exadata Database Machine: Proving 160 Xeon E7 Cores Are As “Slow” As 128 Xeon E5 Cores?

Reading Data Sheets
If you are in a position of influence affecting technology adoption in your enterprise you likely spend a lot of time reading data sheets from vendors.  This is just a quick blog entry about something I simply haven’t taken the time to cover even though the topic at hand has always be a “problem.” Well, at least since the release of the Oracle Exadata Database Machine X2-8.