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Author: Kevin Feasel

Shrinking TempDB

Tara Kizer shows how to shrink a recalcitrant tempdb:

I came across this solution recently when I had to shrink tempdb. I tried shrinking each of the 8 data files plus CHECKPOINTs, repeatedly. It would not budge. I almost threw in the towel and emailed out that the space issue would be fixed during our next patching window, but then I found David Levy’s reply. DBCC FREEPROCCACHE worked like a charm.

Word of warning:  understand what FREEPROCCACHE does before running it.  In an emergency like the scenario Tara describes, the benefit outweighs the cost, but do be aware that there is a cost.

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Columnstore Index Compression Delay

Niko Neugebauer found a “compression delay” option on columnstore indexes in SQL Server 2016 CTP 3.2:

In SQL Server 2016 the OLTP Systems have received a significant improvement – support for the Columnstore Indexes (disk-based Nonclustered Columnstore & In-memory based Clustered Columnstore).
In both cases we have as the base the underlying OLTP-style table, with a Delta-Store object (or Tail Row Group for InMemory tables), that will hold the new data being inserted or updated by the final users. The data that is being frequently updated in OLTP-style systems is called Hot Data. The data that just being inserted into your table is definitely a Hot Data.
The important moment for the table is when the data becomes Cold or mostly infrequently read-accessed, and meaning that it can be compressed into Columnstore format.

This does seem interesting and can be very helpful in using columnstore indexes across different data patterns.

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Scaling Azure SQL Database

James Serra has a post on scaling Azure SQL Database:

Horizontal scaling refers to adding or removing databases in order to adjust capacity or overall performance. This is also called “scaling out”.  Sharding, in which data is partitioned across a collection of identically structured databases, is a common way to implement horizontal scaling.

Vertical scaling refers to increasing or decreasing the performance level of an individual database—this is also known as “scaling up.”

It’s not free and application changes might be required (especially for horizontal scaling), but scaling with Azure SQL Database is pretty straightforward.

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SQL Server 2016 CTP 3.3 Available

CTP 3.3 is now generally available:

In SQL Server 2016 CTP 3.3, available for download or in an Azure VM today, some of the key improvements include:

  • Continued enhancement of Stretch Database: Stretch Database allows you to stretch operational tables in a secure manner into Azure for cost-effective historic data availability. CTP 3.3 includes multiple improvements to Stretch Database, including Azure Stretch database edition preview with support for up to 60TB, Point-in-time restore and geo-failover support.

  • Enhancements to In-Memory OLTP: In-Memory OLTP, which dramatically improves transaction processing performance, has added support in CTP 3.3.

  • Enhancements to Analysis Services DirectQuery models: Analysis Services Tabular Models running in DirectQuery mode now also allows us of DAX filters when defining roles and creation of calculated columns.

  • Enhancements to the new Reporting Services web portal: An updated preview of the new web portal now enables you to add the KPIs and reports you use to your Favorites, to create and edit shared data sources for your KPIs and reports, and to perform other management tasks.

Admittedly, none of those strikes me as compelling “must-download” reasons but the technical overview does have some more details.

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Reversing Sort Order

Michael Swart shows how reversing index sort order can expose invalid assumptions in code:

Remember that this is an application problem and is not a SQL problem. We only get into trouble when applications (or people) expect results to be sorted when they’re not. So unless you have a tiny application, or a huge amount of discipline, it’s likely that there is some part of your application that assumes sorted results when it shouldn’t.

Here’s a method I used that attempts to identify such areas, exposing those assumptions. It involves reversing indexes.

It’s an interesting idea to try out in a dev environment.

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Keep Predicates Meaningful

Gail Shaw shows that adding a valueless predicate to change a scan operation into a seek operation does not guarantee a performance improvement:

I remember a forum thread from a while back. The question was on how to get rid of the index scan that was in the query plan. Now that’s a poor question in the first place, as the scan might not be a problem, but it’s the first answer that really caught my attention.

Since the primary key is on an identity column, you can add a clause like ID > 0 to the query, then SQL will use an index seek.

Technically that’s correct. If the table has an identity column with the default properties (We’ll call it ID) and the clustered index is on that identity column, then a WHERE clause of the form WHERE ID > 0 AND <any other predicates on that table> can indeed execute with a clustered index seek (although it’s in no way guaranteed to do so). But is it a useful thing to do?

Time for a made up table and a test query.

Anything Gail writes is a must-read; this is no exception.

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Visualizing Power BI DMV Queries

Chris Webb shows us how to use Power BI Desktop to visualize DMV queries:

However, running DMV queries against a Power BI Desktop model (which of course runs a local version of the same engine that powers Analysis Services Tabular and Power Pivot) and more importantly doing something useful with the information they return, isn’t straightforward. You can run DMV queries from DAX Studio but that will only give you the table of data returned; you need to copy and paste that data out to another tool to be able to analyse this data. Instead it’s possible to use Power BI Desktop’s own functionality for connecting to Analysis Services to connect to its own local data model and run DMV queries.

It looks like there are some limitations to this technique, but for quick and dirty work, it works.

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Partitioning Thoughts

Kendra Little has a few questions to ask before you set up sliding-window partitioning:

Map this out before you write the code. When will the jobs run, and what should happen if they fail? Should someone be engaged? What tools will they need, and when is the Service Level Agreement for when the process has to be complete? You’ll need lots of details on this to make sure your automation and documentation meet the bar.

Partitioning is an extra layer of complexity.  It can be a very useful extra layer of complexity, but this is a case where it’s best to spend an hour before you begin and walk through potential issues.  Those potential issues will come—automation jobs will fail, external configuration changes will affect your partition strategy, bad data will sneak in and fill up your supposedly-empty edge partitions.

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Check Endpoint Security

Erik Darling ran into an issue with endpoint security while setting up mirroring:

This is the error text:

The ALTER DATABASE command could not be sent to the remote server instance ‘TCP://ORACLEDB.darling.com:5022’. The database mirroring configuration was not changed. Verify that the server is connected, and try again.

SUPER SLEUTH

Alright, that’s silly. I used the GUI. Instead of going to bed I’ll spend some time checking all my VM network settings. BRB.

I’m back. They were all correct. I could ping and telnet and set up linked servers and RDP. What in the name of Shub-Niggurath is going on with this thing?

These things always happen right before bed, right before the big meeting, right before lunch.  They never happen on a slow Tuesday afternoon, it seems…

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