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Curated SQL Posts

Buffer Pools And Availability Groups

Published 2016-07-01 by Kevin Feasel

Joey D’antoni answers the question, what happens to pages in the buffer pool when you have an AG failover?

Recently at SQL Saturday Philadelphia, we started discussing failover  as it relates to mirroring and Always On Availability Groups. Specifically, we were wondering what would happen if you had a relatively busy readable secondary replica (which would have a lot of pages in the buffer pool on the secondary instance) and if those pages would be flushed from cache or anything like that. So I reached out to the product group and Kevin Farlee from Microsoft was extremely helpful

The answer is not what I was expecting.

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Kill Remote Processes

Published 2016-07-01 by Kevin Feasel

Michael Bourgon has a Powershell script to kill remote processes:

Simple problem: we had to replace the config files for an app that hits the database, pointing it at a CNAME.

However, the file could be in use because the app was running.  And the app only loads the file on startup.  So we came up with this, which worked quite well.  Note that we use WMI (see my other posts on it!) to grab the application info then a WMI method to terminate the app.

Click through for the script.

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Filtered Indexes For Uniqueness

Published 2016-07-01 by Kevin Feasel

Shane O’Neill answers one of my favorite interview questions:

I used to think that this would be a complex requirement, possibly requiring aTRIGGER or two to check the inserted value against whatever is already there; but there is a way to have this functionality and have it the way that SQL Server normally would enforce a uniqueness on a column; by using a UNIQUE INDEX.

In case you’re thinking…

“Oh, a unique index doesn’t check what’s already there, is that it?”

I’m afraid that’s not the case.

This is one of my favorite uses of filtered indexes:  “limited” uniqueness.  In other words, I’m okay with an unlimited number of NULL values but all non-NULL values need to be unique.

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New Powershell Cmdlets

Published 2016-07-01 by Kevin Feasel

Rob Sewell looks into some new Powershell cmdlets for SQL Server management:

Chrissy LeMaire has written about the new SQL Agent cmdlets

Aaron Nelson has written about the new Get-SqlErrorLog cmdlet

Laerte Junior has written about Invoke-SQLCmd

All four of us will be presenting a webinar on the new CMDlets via thePowerShell Virtual Chapter Wed, Jul 06 2016 12:00 Eastern Daylight Time If you cant make it a recording will be made available on YouTube on the VC Channel https://sqlps.io/video

There are 17 new Always Encrypted cmdlets and 25 new cmdlets in total.

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Hive With LLAP

Published 2016-06-30 by Kevin Feasel

Carter Shanklin looks at Hive 2’s performance improvements:

LLAP KEY BENEFITS

  • LLAP enables as fast as sub-second query in Hive by keeping all data and servers running and in-memory all the time, while retaining the ability to scale elastically within a YARN cluster.

  • LLAP, along with Apache Ranger enables fine-grained security for the Hadoop ecosystem, including data masking and filtering, by providing interfaces for external clients like Spark to read.

  • LLAP is great for cloud because it caches data in memory and keeps it compressed, overcoming long cloud storage access times and stretching the amount of data you can fit in RAM.

This sounds very much like a response to Spark.

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Biml Text Nuggets

Published 2016-06-30 by Kevin Feasel

Meagan Longoria has started a series on Biml code nuggets and has started with text nuggets:

Text nuggets evaluate the expression they contain and then replace the text nugget with the string representation of the value of the expression. I use them often to switch out names of packages, tasks, and components as well as source and destination tables in SSIS development when creating packages based upon a design pattern.

Text nuggets start with <#= and end with #>. Notice there is an equals sign at the beginning of the text nugget but not at the end.

Text nuggets are very useful. You can include complex business logic in the expressions. And the expression result can be any data type. The BimlScript compiler will automatically convert it to text before replacing the code nugget with the result. Like all code nuggets, text nuggets can be a single line or multiple lines.

Read the whole thing.

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Dealing With Foreign Keys

Published 2016-06-30 by Kevin Feasel

Kenneth Fisher discusses foreign key constraints:

I have to be certain to delete from OrderDetail first, then Order (to maintain the RI) and then load Order first then OrderDetail. No big deal in this simple example, but what if I’m dealing with a dozen tables? Or I’m only re-loading the parent (Order)?

The easiest thing to do is to disable the foreign key, load your data, and then re-enable the foreign key. You might be tempted to skip that last step but don’t. RI is very important and in fact a trusted foreign key can be used by the optimizer to improve your query plan. It’s easy to say that the application doesn’t have bugs that cause problems with referential integrity. It’s not like you’re ever going insert an OrderDetail without an Order right? Unfortunately it’s far to easy for mistakes to happen. Maybe not in the application, it might be a mistake in an update meant to fix something else. Our job is to protect the data, and RI is an important part of that. So by all means disable a foreign key to help with a load but make sure you turn it back on when you are done.

If a foreign key constraint isn’t trusted, the optimizer won’t be able to assume relational integrity, and so it’s possible that the optimizer could make sub-par choices when joining tables with a foreign key constraint.

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Getting Started With Spark

Published 2016-06-30 by Kevin Feasel

I discuss getting up and running with Databricks Community Edition:

There are a couple of notes with these clusters:

  1. These are not powerful clusters.  Don’t expect to crunch huge data sets with them.  Notice that the cluster has only 6 GB of RAM, so you can expect to get maybe a few GB of data max.

  2. The cluster will automatically terminate after one hour without activity.  The paid version does not have this limitation.

  3. You interact with the cluster using notebooks rather than opening a command prompt.  In practice, this makes interacting with the cluster a little more difficult, as a good command prompt can provide features such as auto-complete.

Databricks Community Edition has a nice interface, is very easy to get up and running and—most importantly—is free.  Read the whole thing.

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Too Much NUMA

Published 2016-06-30 by Kevin Feasel

Denny Cherry discusses the performance downsides of having too many NUMA nodes on a server:

At one client I was working with in 2015 they had a server which was configured very similarly to what I’ve described above. They had 32 cores, with 11 NUMA nodes. One NUMA node has 4 cores, the next 9 had three cores each, while the 11th NUMA node had one core in it.

When monitoring the performance on the server what we saw was that three cores were running at 100% (or close to it) while the other cores were running at about 4%.

The reason for this, is that SQL Server is NUMA aware, and it was trying to get the best performance possible out of the configuration that it had access to.

Moderation in everything.  Also, it’s important to plan growth and check every once in a while for oddities like this.

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Visiting Production

Published 2016-06-30 by Kevin Feasel

Randolph West discusses production access:

During a recent client meeting about a database migration, I realised that I have never logged into a SQL Server on their production environment. My involvement has been strictly dealing with setting up the new environment and log shipping the backups.

I get that I’m not a full-service DBA for this client, but it got me wondering about the many security discussions I’ve seen and participated in, in the past: that not even a junior DBA might need access to production database systems, if it’s not within the scope of his or her work.

Limiting production access is a smart move, but it’s important to realize the downstream consequences:  the people who still have access to production will (at least in the short term) have to perform a lot of the tasks that others were doing previously, including data fixes, research, etc.  It’s important to be prepared for that.

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