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Category: T-SQL

Implementing GREATEST in SQL Server 2019

Published 2022-01-04 by Kevin Feasel

Ronen Ariely is on a mission to be the greatest:

The function GREATEST returns the maximum value from a list of one or more expressions. It returns the data type with the highest precedence from the set of types passed to the function.

This function was added to Azure SQL. At this time, it is supported in Azure SQL Database, Azure SQL Managed Instance and Azure Synapse Analytics serverless. 

Unfortunately, it not yet supported on SQL Server on premises and synapse dedicated sql pool.

Click through for a pair of alternative constructs while we wait for GREATEST on-premises.

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Tower of Hanoi in T-SQL

Published 2021-12-29 by Kevin Feasel

Tomaz Kastrun would like to play a game:

T-SQL Code for the popular game of “Tower of Hanoi”, that can be played in Microsoft SQL Server, Azure Data Studio or any other T-SQL editor with support of query execution.

Given that this is the game you use to teach students recursion, I figured a T-SQL based solution would be interesting. Well, Tomaz has the solution and the workspace to play it yourself.

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Use TOP instead of SET ROWCOUNT

Published 2021-12-21 by Kevin Feasel

Jared Poche explains why the TOP clause is superior to using SET ROWCOUNT:

I was presenting on how to use the TOP clause to break down large operations into short, fast, bite-sized operations. The mechanics are things I learned from writing processes that do garbage collection, backfill new columns, and anonymizing PII data on existing tables. I’ve just posted the slides and example scripts here if you are interested.

ARE THEY THE SAME?

The question was whether the SET ROWCOUNT command would work just the same, and the answer is sometimes yes but largely no.

Read on to see what Jared means.

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SERVERPROPERTY() and DATABASEPROPERTYEX() Views

Published 2021-12-20 by Kevin Feasel

Andy Mallon provides a public service:

The thing I hate the most about these two functions is that you need to know the right magic spells to make them work. Let’s look at SERVERPROPERTY() first. The syntax for the function is SERVERPROPERTY( 'propertyname' ), which is easy enough syntax, but the list of values for propertyname isn’t discoverable from SQL Server metadata, DMVs, or even IntelliSense. Instead, I need to check the docs for the list of allowable values. These property names are essentially magic words, and I need to check my spell book to make sure I get it right.

Invalid values for propertyname just return NULL–which is easy enough to handle, but also means your code will compile and run, but might do unintended things if you get your magic spell wrong, due to a typo.

Click through for Andy’s solution to the problem.

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Matching Supply with Demand using Batch Mode

Published 2021-12-17 by Kevin Feasel

Joe Obbish has a solution:

Itzik Ben-Gan posted an interesting T-SQL challenge on SQL performance dot com. I’m writing up my solution in my own blog post because I have a lot to say and getting code formatting right can be tricky in blog post comments. For reference, my test machine is using SQL Server 2019 CU14 with an i7-9700K CPU @ 3.60 GHz processor. The baseline cursor solution completes in 8465 ms on my machine.

Click through for a bit of formal logic and a lot of tuning.

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Hypothetical Indexes in SQL Server

Published 2021-12-15 by Kevin Feasel

Eitan Blumin explains what hypothetical indexes are and why they might be useful:

Using Hypothetical Indexes, you can generate an estimated execution plan for a given query, that would essentially assume the existence of a “hypothetical” index as if it actually exists as a real index. Compare that estimated execution plan to its counterpart without the hypothetical index, and you’ll be able to determine whether creating this index for real is worth the time and effort.

Hypothetical Indexes are actually nothing new in SQL Server. It existed since SQL Server version 2005. However, its use is still not widespread to this day. Most likely because it’s not very easy to use and the relevant commands are undocumented.

Click through to see how to use them and an important warning if you try it in production.

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Query Variation in QDS Toolbox

Published 2021-12-15 by Kevin Feasel

Jared Poche looks at a procedure to track query variation over time:

Since Query Store stores statistics long term, we can use it to look for trends or major changes in performance. The Query Variation component of the QDS Toolbox helps us review changes and has a lot of options to allow us to select the kinds of changes we are interested in.

So, let’s review our options and go through some examples.

Click through for some examples of how to use the query variation procedure.

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Strongly Type those Parameters

Published 2021-12-14 by Kevin Feasel

Erik Darling has a recommendation:

When working with ORMs, care has to be taken to strongly type your parameters to match the data type, length, precision, and scale of the columns those parameters will be compared to. Time and time again, I see the same patterns with string parameters:

– They’re unnecessarily typed as Unicode/nvarchar

– They’re not defined with an appropriate length

– They’re used as catch-all parameters for temporal types (dates, etc.)

Spoiler: these aren’t benefits.

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