SQL Server concepts

How to put tempdb on your Azure VM temp disk

/ Daniel Hutmacher / 9 Comments

Almost all Azure virtual machine sizes come with a temporary disk. The temporary disk is a locally attached SSD drive that comes with a couple of desirable features if you’re installing a SQL Server on your VM:

  • Because it is locally attached, it has lower latency than regular disks.
  • IO and storage are not billed like regular storage.

As the name implies, the temporary disk is not persistent, meaning that it will be wiped if you shut down your VM or if the VM moves to another VM host (as part of maintenance or troubleshooting). For that reason, we never want to put anything on the temporary disk that we need to keep.

tempdb could be a good fit

Since tempdb is wiped and recreated every time we start a SQL Server instance, it could be a great candidate to have on the temporary drive, provided a few prerequisites are met:

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Secure your temporal table history

/ Daniel Hutmacher / 3 Comments

You may have already discovered a relatively new feature in SQL Server called system-versioned temporal tables. You can have SQL Server set up a history table that keeps track of all the changes made to a table, a bit similar to what business intelligence people would call a “slowly changing dimension”.

CREATE SCHEMA App;

CREATE TABLE App.Customers (
    Company_ID      int IDENTITY(1, 1) NOT NULL,
    CompanyName     nvarchar(250) NOT NULL,
	Email           varchar(250) NOT NULL,
	Valid_From      datetime2(7) GENERATED ALWAYS AS ROW START NOT NULL,
	Valid_To        datetime2(7) GENERATED ALWAYS AS ROW END NOT NULL,
    CONSTRAINT PK_Customers PRIMARY KEY CLUSTERED (Company_ID),
	PERIOD FOR SYSTEM_TIME (Valid_From, Valid_To)
) WITH (SYSTEM_VERSIONING=ON);

What happens behind the scenes is that SQL Server creates a separate table that keeps track of previous versions of row changes, along with “from” and “to” timestamps. That way, you can view the contents of the table as it was at any given point in time.

But how to you version the contents of a table, while hiding things like deleted records from prying eyes?

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DATEDIFF performs implicit conversions

/ Daniel Hutmacher / 3 Comments

As I was performance tuning a query, I found that a number of date calculation functions in SQL Server appear to be forcing a conversion of their date parameters to a specific datatype, adding computational work to a query that uses them. In programming terms, it seems that these functions do not have “overloads”, i.e. different code paths depending on the incoming datatype.

So let’s take a closer look at how this manifests itself.

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About wildcards and data type precedence

/ Daniel Hutmacher / 7 Comments

Implicit conversions in SQL Server follow a specific, predictable order, called data type precedence. This means that if you compare or add/concatenate two values, a and b, with different data types, you can predict which one will be implicitly converted to the data type of the other one in order to be able to complete the operation.

I stumbled on an interesting exception to this rule the other day.

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Set up access to network shares from SQL Server

/ Daniel Hutmacher / 4 Comments

Using a local service account for your SQL Server service, your server won’t automatically have permissions to access to other network resources like UNC paths. Most commonly, this is needed to be able to perform backups directly to a network share.

Using a domain account as your SQL Server service account will allow the server to access a network share on the same domain, but if the network share is not on your domain, like an Azure File Share, you need a different solution.

There’s a relatively easy way to make all of this work, though.

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How to add “created” and “updated” timestamps without triggers

/ Daniel Hutmacher / 16 Comments

You have a table that you want to add “created” and “updated” timestamp columns to, but you can’t update the application code to update those columns. In the bad old times, you had to write a trigger to do the hard work for you. Triggers introduce additional complexity and potentially even a performance impact.

So here’s a nicer way to do it, trigger-free.

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How MERGE on two different rows can still deadlock you

/ Daniel Hutmacher / 12 Comments

I recently ran into a curious deadlock issue. I have a process that performs a lot of updates in a “state” table using multiple, concurrent connections. The business logic in the application guarantees that two connections won’t try to update the same item, so we shouldn’t ever run into any locking issues. And yet, we keep getting deadlocks.

What’s going on here? Hint: it has to do with isolation levels and range locks.

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Connecting a SQL Server client on Linux using Active Directory authentication

/ Daniel Hutmacher / 3 Comments

or: How I learned to stop worrying, and love all-caps domain names.

I’m a complete beginner at Linux, so I should preface this post with the fact that these are my humble notes after hours of pulling my hair. It’s not really a fully-fledged how-to article, and there are lot of things I’m not covering. But I figured it may help someone out there at some point.

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