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January 2016

Subquery Effects

Towards the end of last year I used a query with a couple of “constant” subqueries as a focal point for a blog note on reading parallel execution plans. One of the comments on that note raised a question about cardinality estimates and, coincidentally, I received an email about the cost calculations for a similar query a few days later.

Unfortunately there are all sorts of anomalies, special cases, and changes that show up across versions when subqueries come into play – it’s only in recent versions of 11.2, for example, that a very simple example I’ve got of three equivalent statements that produce the same execution plan report the same costs and cardinality. (The queries are:  table with IN subquery, table with EXISTS subquery, table joined to “manually unnested” subquery – the three plans take the unnested subquery shape.)

MobaXterm 8.5

MobaXterm 8.5 was released at then end of last year. The download and changelog can be found here.

If you use a Windows desktop and you ever need to SSH a server or use X emulation, you need this bit of software in your life. There is a free version, or you can subscribe to support the development.

Give it a go. You won’t regret it.

Cheers

Tim…

KeePass 2.31

KeePass 2.31 has been released. You can download it from here.

You can read about how I use KeePass and KeePassX2 on my Mac, Windows and Android devices here.

Cheers

Tim…


KeePass 2.31 was first posted on January 11, 2016 at 9:05 am.
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Public Appearances H1 2016

Here’s where I’ll hang out in the following months:

26-28 January 2016: BIWA Summit 2016 in Redwood Shores, CA

10-11 February 2016: RMOUG Training Days in Denver, CO

Oracle Linux : UEK4 Released

linux-tuxI wrote a post a couple of months ago called
Which version of Oracle Linux should I pick for Oracle server product installations? One of the points I raised was the use of UEK allows you to have all the latest kernel goodies, regardless of being on an older release, like OL6.

I saw a post today about the release of UEK4, so now you have access to all the improvements in the 4.1 mainline Linux kernel, whether you are on are running OL6 or OL7. That just goes to prove the point really.

The “Two Spaces After a Period” Thing

Once upon a time, I told my friend Chet Justice why he should start using one space instead of two after a sentence-ending period. I’m glad I did.

Here’s the story.

When you type, you’re inputting data into a machine. I know you like feeling like you’re in charge, but really you’re not in charge of all the rules you have to follow while you’re inputting your data. Other people—like the designers of the machine you’re using—have made certain rules that you have to live by. For example, if you’re using a QWERTY keyboard, then the ‘A’ key is in a certain location on the keyboard, and whether it makes any sense to you or not, the ‘B’ key is way over there, not next to the ‘A’ key like you might have expected when you first started learning how to type. If you want a ‘B’ to appear in the input, then you have to reach over there and push the ‘B’ key on the keyboard.

Monitor the dNFS activity as of 11.2.0.4

As you may know, there is some views available since 11g to monitor the dnfs activity:

  • v$dnfs_servers: Shows a table of servers accessed using Direct NFS.
  • v$dnfs_files: Shows a table of files now open with Direct NFS.
  • v$dnfs_channels: Shows a table of open network paths (or channels) to servers for which Direct NFS is providing files.
  • v$dnfs_stats: Shows a table of performance statistics for Direct NFS.

One interesting thing is that the v$dnfs_stats view provides two new columns as of 11.2.0.4:

CTEs and Updates

An important target of trouble-shooting, particularly when addressing performance problems, is to minimise the time and effort you have to spend to get a “good enough” result. A recent question on the OTN database forum struck me as a good demonstration of following this strategy; the problem featured a correlated update that had to access a view 84 times to update a small table; but the view was a complex view (apparently non-mergeable) and the update took several hours to complete even though the view, when instantiated, held only 63 rows.

The OP told us that the query “select * from view” took seven minutes to return those 63 rows, and wanted to know if we could find a nice way to perform the update in (approximately) that seven minutes, rather than using the correlated update approach that seemed to take something in the ballpark of 7 minutes per row updated.

Enterprise Manager Cloud Control 13c Upgrade

em-12cA couple of weeks ago I posted about doing a fresh installation of Enterprise Manager Cloud Control 13c (article, blog post). I’ve finally got around to doing an upgrade test from EM CC 12cR5 to 13cR1. You can see the result of that here.

Oracle XE 12c?

There was a post on Oracle-L asking about Oracle Express Edition (XE) 12c. I started to write a reply, but thought a blog post may be more appropriate.

Oracle XE 12c doesn’t exist yet, but people at OpenWorld 2015 confirmed they “plan” to have one. As always, no promises. So when will it arrive? Typically the XE version is put together based on the the first major patchset of release 2 of a version. So the kind of thing you might expect is,

  • 12.2.0.1 : Released mid 2016 maybe.
  • 12.2.0.2 : Released mid 2017 maybe.
  • XE team brought together: Some time after 12.2.0.2 release.
  • XE Released: When it is built and stable.

Things to consider, based on stuff I’ve heard over the last few years.