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August 2016

Month End

A question about parallel query and cardinality estimates appeared on OTN a little while ago that prompted me to write this note about helping the optimizer do the best job with the least effort.  (A critical point in the correct answer to the original question is that parallel query may lead to “unexpected” dynamic sampling, which can make a huge difference to the choice of execution plans, but that’s another matter.)

The initial cardinality error in the plan came from the following predicate on a “Date dimension” table:

Why Mask Data

I’ve been involved in two data masking projects in my time as a database administrator.  One was to mask and secure credit card numbers and the other was to protect personally identifiable information, (PII) for a demographics company.  I remember the pain, but it was better than what could have happened if we hadn’t protected customer data….

Nested Loop Join Physical I/O Optimizations

Having done my mini-series on Nested Loop join logical I/O optimizations a while ago I unfortunately never managed to publish anything regarding the Nested Loop join physical I/O optimizations, which are certainly much more relevant to real-life performance.Therefore the main purpose of this blog post is to point you to Nikolay Savvinov's (whose blog I can recommend in general) great mini-series covering various aspects of these optimizations:Part 1Part 2Part 3SummaryOne point that - at least to me - isn't entirely clear when reading Nikolay's series is which specific plan shape he refers to, in particul

This Autumn, I am mostly being a Conference Tart.

The first half of this year was a little quiet for me on the presenting front. I was concentrating on writing and also on organising events, as opposed to going to them, so most of my trips were for personal reasons (that means “holidays”…). I presented at the Ireland conference and a few UK user group events but that was it – quite a few European events this spring fell on dates I was not available (including the Israeli and Finnish conferences where I was asked to attend and would have loved to). Or, oh the shock of it, my submissions were not accepted! {How dare they</p />
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Syntax formatter might change your data

I saw this on an AskTom question today answered by my colleague Chris.  Check out this simple example


SQL> create table T (
  2    x int default 1,
  3    y int default 1
  4   ,z int);

Table created.

It looks like I’ve assigned a default of “1” to both X and Y. But lets now dump out the default definition from the dictionary.

Data Exposure, leakage and Reporting

I have had an interesting few interactions over the last week or so regarding data supposedly leaked from my website. This is interesting from two perspectives. The first is that three people emailed me and told me that my website....[Read More]

Posted by Pete On 10/08/16 At 10:23 AM

Uber’s move to MySQL

I was reading a very interesting article on Uber’s move from Postgres to MySQL. I really like it when IT professionals and/or companies take the time to explain their technology decisions.  It’s a brave thing to do, because it’s easy for people to jump on the bashing bandwagon (“Ha ha … Company X chose Y and now they’re bust” etc etc).  It’s the same reason you rarely see detailed customer reference information about the technology they are using, or how they are succeeding or failing.  It’s generally kept pretty quiet.  So for Uber engineering to be open about it is impressive, and a lesson for us all.

Not being as familiar with either Postgres or MySQL, one statement really caught my attention (colour emphasis mine):

Yes, Host Aggregate I/O Queue Depth is Important. But Why Overdo It When Using All-Flash Array Technology? Complexity is Sometimes a Choice.

Blog Update. Part II is available. Please Click the following link after you’ve finished this post: click here.

That’s The Way We’ve Always Done It

I recently updated the EMC best practices guide for Oracle Database on XtremIO. One of the topics in that document is how many host LUNs (mapped to XtremIO storage array volumes) should administrators use for each ASM disk group. While performing the testing for the best practices guide it dawned on me that this topic is suitable for a blog post. I think too many DBAs are still using the ASM disk group methodology that made sense with mechanical storage. With All Flash Arrays–like XtremIO–administrators can rethink the complexities of they way they’ve always done it–as the adage goes.

Index Competition in #Oracle 12c

Suppose you want to find out which type of index is best for performance with your workload. Why not set up a competition and let the optimizer decide? The playground:

Oracle Security Talks, Training and Conferences

Kamil Stawiarski who runs Database Whisperers sp. z o. o. sp. k., an Oracle specialist consulting company in Poland and whose company is also a reseller for our Oracle database security scanner PFCLScan in Poland has invited me to speak....[Read More]

Posted by Pete On 08/08/16 At 12:48 PM