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May 2014

Head to Head with Rick

Great guy; Great concert (Royal Albert Hall, April 30, 2014). With Rick in a “Grumpy Old Man Pose” – Nuff Said!

NFS Performance Issues at TCP level

What happens with I/O requests over NFS and more specifically with Oracle? How does NFS affect performance and what things can be done to improve performance?

What happens at the TCP layer when I request with dd an 8K chunk of data off an NFS mounted file system?

Here is one example:

I do a

dd if=/dev/zero of=foo bs=8k count=1

where my output file is on an NFS mount, I see the TCP send and receives from NFS server to client as:

(the code is in dtrace and runs on the server side, see #2970a6;" href="https://sites.google.com/site/oraclemonitor/tcp-d">tcp.d for the code)

#2970a6;" href="http://dboptimizer.com/wp-content/uploads/2011/04/nfs3.png">

Right Deep, Left Deep and Bushy Joins in SQL

#555555;">What is right deep verses left deep? Good question. In join trees (not VST) the object on the left is acted upon first then the object on the right.  Below are left deep and right deep examples of the same query, showing

    #555555;">
  • query text
  • join tree
  • join tree  modified to more clearly show actions
  • VST showing the same actions

#555555;">left_right_deep copy

Richard Foote’s Art Work (Art Decade)

Just a quick note to say I have a new tab on the blog called “Richard Foote’s Art” where I’ll periodically upload a new piece of my art work. As the page says, they highlight both what’s in my mind (so beware) and how I like to relax in the evenings. All pieces are for […]

Extended stats

Like the recent article on deleting histograms this is another draft that I rediscovered while searching for some notes I had written on a different topic – so I’ve finally finished it off and published it.

Here’s a quirky little detail of extended stats that came up in an OTN thread earlier on this week [ed: actually 8th Jan 2014]. When you create column group stats, Oracle uses an undocumented function sys_op_combined_hash() to create a hash value, and if you gather simple stats on the column (i.e. no histogram) you can get some idea of the range of values that Oracle generates through the hash function. For example:

12c New Optimizer Features

Besides the officially available information about new optimizer features in 12c it is always a good idea to have a look at the internal optimizer parameters that show what features are enabled when running with OPTIMIZER_FEATURES_ENABLE = 12.1.0.1. Here is the list of internal optimizer parameters and fix controls that are different between 11.2.0.4 and 12.1.0.1:

Optimizer parameters:

#eeeeee; border: 1px dashed rgb(204, 204, 204); overflow: auto;">_optimizer_partial_join_eval           partial join evaluation parameter                            
_optimizer_unnest_scalar_sq            enables unnesting of of scalar subquery                     
_optimizer_ansi_join_lateral_enhance   optimization of left/full ansi-joins and lateral views      
_optimizer_multi_table_outerjoin       allows multiple tables on the left of outerjoin             

12c Hybrid Hash Distribution with Skew Detection / Handling

Oracle 12c introduces several new features in the area of Parallel Execution. Over the next couple of weeks I attempt to publish more about them - Jonathan Lewis for example already published a note about the new "PQ Replication" feature that applies to the BROADCAST distribution of small tables.One important new feature is the automatic skew handling for parallel joins. I've already given an overview of the feature in my mini-series "Parallel Execution Skew" at "AllThingsOracle.com", so if all you want is a high-level overview I recommend reading the article there.The purpose of this note here is to provide a few more internals and details about that feature.First, just a short summary of the prerequisites of the feature to work:1.

OAUTH_UTL Roundup

So I had a good run in the first half of the month. I got a package ready, that can do all the steps of the Oauth v. 1.0 protocol flow. I have started the v. 2.0 protocol implementation, but half way through the month, I got sidetracked. So som exciting news might be coming, soon :)

Oracle Alchemist

Steve Karam's Oracle Blog

Upcoming Webinar: Writing SQL Right on May 20

Coming up on May 20, I'll be delivering a webinar entitled Oracle SQL Performance: Writing SQL Right. Registration is now open. Once again the webinar will be hosted by Embarcadero and there will be two sessions - one at 10am ET and one at 2pm ET.


Abstract: