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Day: July 6, 2016

Forcing Predicate Pushdown

Published 2016-07-06 by Kevin Feasel

I have a blog post on some troubles I’ve had with the FORCE EXTERNALPUSHDOWN hint:

As soon as I kick this off, I get an error:

Msg 7320, Level 16, State 110, Line 1
Cannot execute the query “Remote Query” against OLE DB provider “SQLNCLI11” for linked server “(null)”. Query processor could not produce a query plan because of the hints defined in this query. Resubmit the query without specifying any hints.

Well, that’s not good…  Checking sys.dm_exec_compute_node_errors gives me four error rows with stack trace results, none of which seems very useful to me (as opposed to a Microsoft support tech).

I don’t have any good answers in this blog post, so I’m hoping to learn more and report back later.

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SOS_WaitableAddress

Published 2016-07-06 by Kevin Feasel

Ewald Cress looks at the SOS_WaitableAddress class next in his series on internals:

All the synchronisation mechanisms I have discussed so far have one things in common: in order to be globally visible, they involve synchronisation objects embedded within the things they protect. So for instance, if we want to protect the global scheduler list with a spinlock, that spinlock lives within the global scheduler list, and allocating the spinlock’s storage (lightweight as it is) is the responsibility of whoever creates that list. But what if there were millions of things we might occasionally want to lock, and we don’t want to embed a lock within each such item due to the hassle and/or overhead involved? What if we just wanted a publicly visible corkboard upon which we can pin notes describing the items which are currently locked?

This is a rather different locking structure than we’ve seen so far.  Read on for details, including magic within the Signal method.

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Graphing Swear Words In Movies

Published 2016-07-06 by Kevin Feasel

Jos Dirksen uses Spark and D3 to count and graph swear words in movies:

So how do we do this? Well, the first thing to do is get the number of swearwords per minute. I mentioned that for the original article someone just counted every swearwords, in our case, we’re just going to parse a subtitle file, and extract the swear words from that.

Without going into too much detail, you can find the code I’ve experimtend with in this gist (it’s very ugly code, since I just hacked something together that worked).

Jos includes counts for four movies.  This link does contain a few bad words, but if you get past that, it’s a good pattern for analyzing word counts in general.

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TDE And Backup Compression

Published 2016-07-06 by Kevin Feasel

Erik Darling notes that databases using Transparent Data Encryption now support backup compression:

First, the database without a Max Transfer Size at the bottom was a full backup I took with compression, before applying TDE. It took a little longer because I actually backed it up to disk. All of the looped backups I took after TDE was enabled, and Max Transfer Size was set, were backed up to NUL. This was going to take long enough to process without backing up to Hyper-V VM disks and blah blah blah.

The second backup up, just like the blog man said, no compression happens when you specify 65536 as the Max Transfer Size.

You can see pretty well that the difference between compressed backup sizes with and without TDE is negligible.

Check it out, including the table Erik put together.  I’m glad that backup compression is now supported, although I’m kind of curious how they can do that while retaining encrypted backups—are they decrypting data, writing to backup (and compressing), and then encrypting the backup?  That’d be worth checking out with a hex editor.

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“Add Existing Package” Quirk

Published 2016-07-06 by Kevin Feasel

Kenneth Fisher points out an “interesting” quirk with SQL Server Data Tools when you try to add an existing package under certain circumstances:

That SSIS change just won’t stick! There is a new business requirement, so now you have to change an existing SSIS package. You opened up a new project, imported the package, and made your changes.

But the changes just aren’t showing up. You back and look again and sure enough, the changes you made are there. You run your job (or whatever) and still the change aren’t taking effect. So what happened?

Read on for the cause and the solution.

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SET XACT_ABORT On By Default

Published 2016-07-06 by Kevin Feasel

Tibor Karaszi notes an important change to SQL Server Management Studio with the July 2016 release:

Now, exposing one more SET option can’t be bad, right? But Erland Sommarskog found out that this is checked by default. Now, using XACT_ABORT might now [sic] be a bad thing, au contraire. See this one of Erland’s series of error handle articles.

Making this change after all of these years is a little odd.  Making it in a regular update is very odd.  Using SET XACT_ABORT ON is a smart move in general, but there are times in which you don’t want to rollback immediately after an error; the problem is, are all of those places in your code well-documented?

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New Powershell Cmdlets

Published 2016-07-06 by Kevin Feasel

Chrissy LeMaire talks about the 25 new cmdlets in SQL Server’s Powershell module:

One of the best things about these cmdlets is that it makes failed jobs a whole lot easier to find. Note that SQL Server Management Studio 2016 does not have to be installed on the server — only your workstation. That’s because the cmdlet is built on top of SMO which is available in all versions of SQL Server since 2000.

As an aside, you may be wondering what a cmdlet is, and what’s the difference between a cmdlet and what you and I write. A cmdlet is a formally written PowerShell command that’s written in C#. I’ve never written one in my life. Instead, I write Functions. They’re both essentially the same, but a cmdlet is fancier on the back end.

Powershell is the present (not the future) for administration, so it’s nice to see the SQL Server team pushing themselves a bit when it comes to cmdlets.  They’ve a long way to go, but this is a nice step.

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Indexes On Disk

Published 2016-07-06 by Kevin Feasel

Kendra Little has a great diagram showing which indexes are disk-based and which are memory-resident:

I was looking through some terms in SQL Server documentation the other day, thinking about what it’s like to learn about SQL Server’s indexes when you’re new to the field. I jotted down a note: B-tree = Rowstore = Disk Based

And then I realized that’s not quite right.

Not all disk based indexes are traditional clustered and nonclustered indexes. Columnstore indexes are also disk based. Updatable Columnstore indexes use special rowstore B-trees behind the scenes. And Books Online says “rowstore” also refers to Memory-Optimized tables.

If you’re new to indexing, this picture will save you some learning time.

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Histograms

Published 2016-07-06 by Kevin Feasel

Devin Knight continues his Power BI visualization course:

In this module you will learn how to use the Histogram, a Power BI Custom Visual.  A Histogram is a column chart which shows the distribution of occurrences divided into categories, called bins.  This type of chart is useful for estimating density and discovering outliers.

Another fine entry in a great series.  Check it out.

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