March 2013

Premature optimization, (probably) because of Donald Knuth’s famous line “premature optimization is the root of all evil,” (see Structured Programming with go to Statements) is, at the very least, a controversial topic. The misconception based on that particular quote is that a programmer, while writing code, should ignore optimization altogether. In my opinion this is wrong. To put the quote in context, let’s have a look to at the text that precedes and follows it:

For all those folks who are forced to use Windows, you might be interested to know UltraEdit v19 has just been released.

You can check out the new features here.

The latest version for Mac/Linux is still 3.3, so maybe we are due a new version there too?

Cheers

Tim…

Take a look at the following Parallel section of a SQL Monitor report:

Any query which produces such a report won't care about how much parallel you're running because virtually all the time is spent by the query coordinator (which is a serial process) being busy.

In this case the query in question is quite simple:

select /*+ parallel(t,8) */ median(basket_amount) from whs.fact_sale t

The reason it behaves the way it does has everything to do with how Oracle executes it:

Execution Plan
----------------------------------------------------------
Plan hash value: 712547042

-----------------------------------------------------------------------------------------------------------------------------------------

In the previous post I've demonstrated an unexpected Nested Loop Join caused by an extreme data distribution. Although unexpected at first sight, the performance of the execution plan selected by the optimizer is decent - provided the estimates are in the right ballpark.Here is another case of an unexpected execution plan, this time about Merge Joins.

Merge Joins

In order to appreciate why the execution plan encountered is unexpected, first a quick summary about how Merge Joins work:A Merge Join is essentially a Nested Loop operation from one sorted row source into another sorted row source.