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Curated SQL Posts

Matching Supply and Demand

Published 2022-03-09 by Kevin Feasel

Itzik Ben-Gan continues looking at interesting solutions to a tricky problem:

Last month I covered solutions based on a revised interval intersections approach compared to the classic one. The fastest of those solutions combined ideas from Kamil, Luca, and Daniel. It unified two queries with disjoint sargable predicates. It took the solution 1.34 seconds to complete against a 400K-row input. That’s not too shabby considering the solution based on the classic interval intersections approach took 931 seconds to complete against the same input. Also recall Joe came up with a brilliant solution that relies on the classic interval intersection approach but optimizes the matching logic by bucketizing intervals based on the largest interval length. With the same 400K-row input, it took Joe’s solution 0.9 seconds to complete. The tricky part about this solution is its performance degrades as the largest interval length increases.

This month I explore fascinating solutions that are faster than the Kamil/Luca/Daniel Revised Intersections solution and are neutral to interval length. The solutions in this article were created by Brian Walker, Peter Larsson, Paul White, and me.

These are some of the best solutions but the whole series has been quite interesting.

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Data Virtualization with Azure SQL Managed Instance

Published 2022-03-09 by Kevin Feasel

Mladen Andzic announces data virtualization in Azure SQL Managed Instance:

Data virtualization capabilities, now in preview in Azure SQL Managed Instance, enable you to execute Transact-SQL (T-SQL) queries against data from files stored in Azure Data Lake Storage Gen2 or Azure Blob Storage and combine it with relational data stored locally in the managed instance using logical joins. This way you can transparently access external data while keeping it in its original format and location. There is no data duplication or need to run and maintain ETL processes, which means that you can extract and deliver insights faster. Currently supported file formats are Parquet, CSV, and JSON.

I’m going to start calling it PolyBase Duck Typing: it’s not actually PolyBase but the syntax is the same and the outcome is the same and the method to enable it is the same and “PolyBase” is a lot easier to say than “data virtualization.” So even though it’s not PolyBase, I’m going to call it PolyBase until there’s a meaningful split.

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Watching and (Not) Messing with Optimization Phases

Published 2022-03-09 by Kevin Feasel

David Alcock giveth:

The full optimisation stage is where the optimiser uses a bag of tricks to optimise our query (surprise, surpise), well technically it has three bags of tricks that are named optimisation phases that each contain a collection of transformation rules (which I cover in this post that you should never do). The optimiser is not limited to using just one of the phases and each has a set criteria which determines if the optimiser can use that particular phase.

In order to see what how the optimiser is using these phases we need to enable Trace Flag 8675 as well as Trace Flag 3604 which will redirect the output to the query messages tab in Management Studio:

And David Alcock taketh away:

Now it has to be said it’s undocumented for a reason, the reason is that it’s really not a good idea to do this. In fact enabling this trace flag is such a bad idea that it will probably cause no end of issues with query performance…so let’s do it, but before we do let me add yet again that please don’t do this! Disabling optimisation features is a really bad idea, just like we did in this post – the purpose for this demo is just to show that we can, and how dangerous it can get.

This is fun to learn and interesting when doing advanced troubleshooting, but maybe not something you want to do very often.

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Preventing Triggers from Firing for a Single Process

Published 2022-03-09 by Kevin Feasel

Andy Mallon builds a trigger guard:

I recently saw a question on DBA Stack Exchange (it has since been deleted by the author), who had a “special process” that ran regularly, and as part of that process, they disabled the trigger, did some stuff, and re-enabled it. During that process, the step that disables the trigger would deadlock, and cause problems. So the asker was wondering how to catch & handle the deadlock during the DISABLE TRIGGER step.

Yeah, disabling the trigger, not so great. Read on for one interesting way of doing it, as well as a few other methods in the comments.

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Trivial Plans and Stats Updates

Published 2022-03-08 by Kevin Feasel

Lonny Niederstadt lays out the harshness of reality:

OK.  SQL Server trivial plans for rowstore table INSERT. And related optimizer stats interaction.
TL;DR cached trivial plans for INSERT can be surprisingly stubborn. If a query matches to one, it won’t perform or queue a stats update even if the stats are stale.  If the stats have been updated and would otherwise warrant a per-index plan – but there is a matching cached trivial plan for a per-row plan… outta luck. Might hafta DBCC FREEPROCCACHE or add OPTION(RECOMPILE) hint to make sure a cached trivial plan doesn’t prevent a per-index update for an INSERT when you really want one.

Read on for a dive into the topic.

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The Power of PIVOT and GROUPING SETS

Published 2022-03-08 by Kevin Feasel

Aaron Bertrand builds a report:

Without comprehensive reporting tools (or Excel), it can be cumbersome and frustrating to produce perfect report output from SQL Server SELECT statement or stored procedures. In modern versions, we have access to T-SQL functionality that far exceeds old-school ROLLUP and CUBE, like PIVOT, UNPIVOT, and GROUPING SETS. Let’s look at how to produce output we can easily plug into a simple front end and produce great-looking reports.

GROUPING SETS is one of my favorite under-utilized operators.

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From Access to SQL Server

Published 2022-03-08 by Kevin Feasel

Tom Collins has some tips to make an Access to SQL Server migration more successful:

-Access has a size limit of 2 GB

-Access has a concurrent users limit of 255 users

-Require increased capacity 

The SQL Server Migration Assistant for Access (SSMA) is a very useful tool  offered by Microsoft . 

The main objective of these notes is to supplement the Microsoft documentation and to assist in Access to SQL Server journey.      

Read on for those notes.

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Stringing Azure Data Factory between VNets

Published 2022-03-08 by Kevin Feasel

Ahmed Mahmoud performs networking wizardry:

Customer wants to connect Azure Data Factory on one subscription to an Azure SQL Server on Virtual Machine (SQL VM) on another subscription. check out the architecture diagram below for more clarification.

Click through for that diagram as well as the process. And between VNet peering and Private Link, I believe (but could be wrong in saying) the traffic would never leave Azure-hosted machines even when it transits between subscriptions.

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Query Plans in Azure Data Studio

Published 2022-03-08 by Kevin Feasel

Grant Fritchey is excited:

I have long been a fan of Azure Data Studio, but one shortcoming has kept me from truly adopting it: Query Plans in Azure Data Studio. Sure, there was a plug-in you could install. Also, you could use a somewhat truncated version of Plan Explorer, but all I wanted was for SQL Server Management Studio plans to be query plans in Azure Data Studio.

Go and get version 1.35 of the tool. Right now.

I think there’s still a fair amount of work to do on those plans but it’s a far cry from where they were prior to this.

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Database Offline Works but Online Permissions Failure

Published 2022-03-08 by Kevin Feasel

David Alcock unravels a mystery:

I was browsing the SQL Server subreddit earlier where someone had posted a problem where they’d been able to take a database offline but couldn’t bring the database back online via a script or the UI in SSMS (full thread here).

There’s a bit of a back story; all the DBA’s have left the business (facepalm) so a non-DBA has been left with the admin type tasks. Secondly the reason the database was being taken offline was to take physical backups of the databases mdf and ldf files (double facepalm).

That is its own issue but read on for the problem at hand.

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