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Category: Documentation

Documenting dm_db_missing_index_group_stats_query

Erik Darling does a good deed:

When I was checking out early builds of SQL Server 2019, I noticed a new DMV called dm_db_missing_index_group_stats_query, that I thought was pretty cool.

It helped you tie missing index requests to the queries that requested them. Previously, that took a whole lot of heroic effort, or luck.

This has long been a pain point for index-based query tuning. You can guess what the types of queries can look like based on the suggested columns, but one risk of that guessing is that index order matters but the columns get returned in the order in which they exist on the table, not necessarily the order in which they would be most useful.

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Power BI Model Documentation

Marc Lelijveld reflects on an overlooked part of development:

I strongly believe that it is key to describe everything that you have built as part of your Power BI model. As we all have a hate-love relationship with documenting our work, the external tool that I build to document your Power BI solutions could come in useful. In particular everything you add and only exists in your model is important to describe properly. This documentation is key once you share your data model with others and they try to understand the goal of a measure, column or anything else. Other than that, documentation also comes in useful if you handover your solution to your colleague or client.

Read on to see what you can currently document.

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Creating Sequence Diagrams for SSIS Packages

Aveek Das has an idea for documentation:

In this article, I am going to explain in detail how to document SSIS packages using Sequence Diagrams and the importance of these diagrams in the field of software engineering, no matter which programming language are you using. In my previous article, I have talked about the various UML Diagrams that are being used to document various software engineering processes. Also, I have talked about modular ETL architecture and how to create such a modular package in SSIS. Sequence diagrams are also a part of the broader UML Diagrams which define the interaction between the various components in the system in a chronological manner.

My gut feeling is that this works best with medium-sized collections of packages, where we’re talking 10-30 or so packages in total, and that for something much larger, I’d want an automated tool to build diagrams for me. But I could be way off base on that.

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DATETIME2 and Storage Size

Randolph West digs into an issue:

Two years ago I wrote a post that got a lot of traction in the comments at the time. Last month there was renewed interest because one of the commenters noted that the official SQL Server documentation for DATETIME2 disagreed with my assertions, and that I was under-representing the storage requirements.

To remind you, I have been saying for years that you can use DATETIME2(3) as a drop-in replacement for DATETIME, and have better granularity (1ms versus 3ms) for 12.5% less storage (1 byte per column per row). The commenter intimated that because my statement conflicted with the documentation, that I must be wrong. As it turns out the documentation was wrong, but I also learned something new in the process!

It’s an interesting internal look at how difficult it is to get documentation right, even on something which sounds simple.

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Power BI Migration Documentation

Melissa Coates announces some new documentation:

I’m really pleased to announce that new Power BI Migration documentation that I authored is published on the Microsoft Docs site. Although it was written from the perspective of migrating to Power BI from another platform, there’s no doubt that a lot of the content applies to a standard Power BI implementation project.

Click through to learn more and follow Melissa’s link to get the whole document.

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Documenting a Power BI Model

Marc Lelijveld has taken advantage of external tools in Power BI:

Yes, really! I build and External Tool to document your Power BI Model. I know, documentation is not a very famous topic, but I believe a very important one! Lead time from data to insights is very short with Power BI, but often we forget to look back on what kind of monster we created. Especially if you want to share the dataset for reuse, I believe it is important to deliver some documentation as well. Maybe you even promote or certify this dataset in the future, which implicates that the model matches certain quality metrics and best practices.

Read the whole thing and check out Marc’s offering.

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Documenting SQL Server Tables

Phil Factor has a way to create table documentation in source control and propagate it to the actual database:

It has always been a problem that documentation in the source, where it should be, is not then passed into the live database when the build script is executed. In a table, you have columns, constraints and indexes that you are likely to document using line-ending comments and block comments. You probably have a big block comment at the start, explaining the table. This information should be available in the live database. Microsoft don’t have a good answer and vaguely go on about adding comments in extended properties. Well, that’s fine but it hasn’t happened, unsurprisingly: Have you ever tried to do it? It is an almost impossible task, even with SQL Doc.

My solution is to execute my finely-documented build script as usual to create the latest version of the database, and then process the same script in PowerShell to add all the comments and documentation as extended properties in the right place in the live database.

It’s an interesting approach to a classic problem.

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Fun with asciidocs

Sheldon Hull explains the value of asciidocs:

Documentation is such an important part of a developer’s life. I think we often take it for granted, and it’s an afterthought in many projects. However, as I consider my work, I know that I’m not reinventing the wheel very often 😀. Most of what I do is built on the back of others’ work. When I use tooling, I’m reading the documentation and using it as my basis to get work done. When I use my notes and blog posts as a reference, I’m using my informal version of knowledge gathering.

INVEST in documenting your work as you go, for the person behind you. You don’t find time to do it, you make time to do it while you work, as a first class citizen of your work, not an after-thought. Think of all the times you’ve had to dig for answers and save someone else that experience.

Sheldon is not wrong.

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Managing SQL Server Documentation with JSON

Phil Factor gives us the gloop:

Metadata extract files are handy for documentation, study, cataloguing and change-tracking. This type of file supplements source because it can record configuration, permissions, dependencies and documentation much more clearly. It is a good way of making a start with documenting your database.

Here is a sample of a json metadata file (from AdventureWorks 2016). It was generated using GloopCollectionOfObjects.sql that is here in Github, and is being viewed in JSONBuddy. I use this format of JSON, a collection of documents representing SQL Server base objects (no parent objects) when I need to read the contents into MongoDB. The term ‘Gloop’ refers to a large query that, you’d have thought, would be better off as a procedure. Here is a typical sample of the output.

This is an interesting approach to documentation. I’m not totally buying into it, but that might just be due to my not having tried it.

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Database Restoration and the Plan Cache

Andy Mallon has some tests for us:

If you restore a database, what does that do to the plan cache? Well, let’s start by looking at the documentation for RESTORE. (Emphasis mine)

Restoring a database clears the plan cache for the instance of SQL Server. Clearing the plan cache causes a recompilation of all subsequent execution plans and can cause a sudden, temporary decrease in query performance. For each cleared cachestore in the plan cache, the SQL Server error log contains the following informational message: ” SQL Server has encountered %d occurrence(s) of cachestore flush for the ‘%s’ cachestore (part of plan cache) due to some database maintenance or reconfigure operations”. This message is logged every five minutes as long as the cache is flushed within that time interval.

Yikes. That first sentence sounds like it is going to clear the cache for the entire instance.

Read on as Andy tests this and (spoiler alert) changes the documentation.

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