daniel – Page 2 – sqlsunday.com

How to put tempdb on your Azure VM temp disk

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:

Secure your temporal table history

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?

I made a new demo/workshop database

For a couple of years now, I’ve taught my SQL Server workshops using a demo database that I constructed way back in 2014. The old demo database I used was modelled on a fictional airline, but the data was made up, and the modelling.. wasn’t great.

Chicago, the city of complicated parking. Photo credit: Sawyer Bengtson on unsplash.com

So I spent a couple of hundred hours to build a new database based on a public dataset from the City of Chicago with parking tickets issued between 1997 and 2017. I’ve also refined this dataset with more details from other public sources – from ZIP codes, to socioeconomic indicators, to weather information and more.

Querying a single table can use multiple indexes

Can SQL Server piece together two different indexes in a single-table query, rather than just giving up and scanning a suboptimal clustered index? The short answer is: yes, in a fairly narrow band of conditions.

Watch out for Merge Interval with date range Index Seeks

In my last post, I found that DATEDIFF, DATEADD and the other date functions in SQL Server are not as datatype agnostic as the documentation would have you believe. Those functions would perform an implicit datatype conversion to either datetimeoffset or datetime (!), which would noticeably affect the CPU time of a query.

Well, today I was building a query on an indexed date range, and the execution plan contained a Merge Interval operator. Turns out, this operator brings a few unexpected surprises to your query performance. The good news is, it’s a relatively simple fix.

DATEDIFF performs implicit conversions

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.

Computing the modulus from very large numbers

… and what of this all has to do with IBAN numbers.

The modulus is the remainder of a division of two integers*. Suppose you divide 12 by 4, the result is 3. But divide 11 by 4, and the result is 2.75. This could also be expressed by saying that 11/4 is 2 with a remainder of 3. Computing that 3 is the work of the modulo operator, which in T-SQL is represented by the % operator.

Let’s explore how to compute the modulus of large numbers in SQL Server, and how this is useful in the real world.

T-SQL template parser

Just for the heck of it, I scratched together a template parser for T-SQL . The usage of this function is similar to the STRING_SPLIT() function, except instead of splitting a string by a delimiter character, we want to split a string according to a defined template.

SELECT *
FROM dbo.Template_Split(
    'Wingtip Toys (Bethel Acres, OK)',
    '% (%, %)'
);

… will generate the following output:

Notice how the “%” wildcard character denotes how the string is split. Unlike the fancy stuff you can do with regular expressions, T-SQL wildcards don’t allow you to define capture groups, so this function is unfortunately constrained to just using “%”. I hope it will still come in handy to someone out there.

That’s it, that’s the post. Enjoy!

Use Ctrl+F1 as a “preview” button in SSMS

Every time I set up SQL Server Management Studio, I take the time to add a shortcut to the “Query Shortcuts” section of the options:

Tools -> Options -> Environment -> Keyboard -> Query Shortcuts

On the surface, these query shortcuts are just what the name implies – a key combination that you can press to run a command or execute a stored procedure. But there’s a hidden super power: whatever text you’ve selected in SSMS when you press the keyboard combination gets appended to the shortcut statement.

So if you select the name of a table, for instance “dbo.Votes”, and press Ctrl+F1, SSMS will run:

SELECT TOP (1000) * FROM dbo.Votes

This allows you to create a keyboard shortcut to instantly preview the contents of a table or view.

And you can select not just the name of one table, but any other query text you want to tack on:

Preview the contents of two joined tables.

Because we’ve selected both the name of a table and the next line, pressing Ctrl+F1 in SSMS will effectively run the following command:

SELECT TOP (1000) * FROM dbo.Votes AS v
INNER JOIN dbo.VoteTypes AS vt ON v.VoteTypeId=vt.Id

You can go on to include as many joins, WHERE clauses, ORDER BY, as long as the syntax makes sense:

Remember that query shortcuts only apply to new windows, so if you change them, you’ll have to open a new window for the change to take effect.