I like free stuff and I like books, so of course I like free books – and it seems that the more I provide free publicity for relevant books here the more free books I get sent. I’ve now got enough to merit writing another post covering those I’ve received recently from various publishers and authors. As always these are not reviews, just short summaries of books you might want to check out.
Studying for a certification is a great way to learn a technology and this book is intended for those studying for the PL-300 Power BI Data Analyst exam. In terms of technical content this book is a good general introduction to Power BI development and administration, so nothing out of the ordinary, but knowing that the book is written to cover the exam syllabus and the generous number of practice questions would be the reason to buy it.
Taking a different angle on learning Power BI is Wyn Hopkins, who has aimed his book at people coming from the Excel community (Wyn himself is a well-known Excel and Power BI MVP who has a great YouTube channel). I’m surprised there aren’t more people writing content like this since the vast majority of people using Power BI come from this background; Rob Collie cornered the market years ago but has been very quiet recently. Once again it’s an introductory guide to Power BI development but there’s a healthy amount of real-world experience inside as well as opinion, which I like – it not only makes the book more valuable but also more readable.
Adam Aspin is a prolific author of Power BI books and this one focuses on the mechanics of building reports and dashboards. It’s not one of those preachy “data visualisation” books but a guide to the Power BI report canvas, all the visuals and their properties and settings: there’s a whole chapter on drilling up and down, for example. As a result even an experienced Power BI report designer will probably find something in it that they didn’t know.
Not strictly a Power BI book, I know, but a general introduction to Synapse in the worked example/cookbook format – although there is a chapter on how to use Power BI with Synapse. Gaurav is a colleague of mine on the Power BI CAT team at Microsoft so of course I want to call out this new book that he has co-written! My Synapse knowledge is not as good as it should be so I learned a few things reading it.
[This post was originally published on the official Power Query blog, which has now been taken down. I’m republishing all my posts there to this blog to ensure the content remains available.]
Yet another new transformation has been added to Power Query Online: you can now add columns containing ranks to a table. To see how to use it, consider the following table containing sales and profit values for various products:
Let’s say you want to add a new column to this table containing the rank of each product, calculated according to its sales. To do this select the Sales column so that it is highlighted (as shown in the screenshot above) and then go to the Add Column tab on the ribbon in Power Query Online and click on the “Rank column” button:
When you do this the following dialog will appear:
Click OK and the dialog will close, and you will see that a new column has been added to your table containing the ranks for each product by sales and that the table itself has been sorted by the values in the Sales column too:
In most cases this will give you the values you want, but by clicking on the Advanced radio button in the Rank dialog you’ll find more options for controlling how the rank is calculated.
First of all, notice that because Oranges and Pears have the same sales value they both have a rank of 2 and that the next product, Apples, has a rank of 4. This is because the default rank method, Standard competition, has been used. There are there are two other methods of calculating ranks though. Choosing the Dense method in the Advanced tab of the dialog from the “Rank method” dropdown:
…changes the rank value for Apples to 3:
Choosing the Ordinal rank method ignores any tied values like so:
A different way of handling tied values for sales is to consider profit values, and this is also possible on the Advanced tab. Clicking the “Add ranking” button allows you to add another column to sort by, so that when sales is tied then the product with the highest profit will get the highest rank:
Here’s the output:
Notice now that Pears has a rank of 2 and Oranges has a rank of 3 because, even though they have the same sales, Pears has a higher value for Profit.
Behind the scenes this new functionality is powered by a new M function called Table.AddRankColumn which Reza Rad has already blogged about in detail here.
Recently I collaborated with a number of people from Microsoft and Snowflake to write a blog post on best practices for using Power BI in DirectQuery mode on Snowflake. You can read it here:
It builds on what is already in the Power BI guidance documentation for DirectQuery to add some advice specific to Snowflake. It also has a few other tips that are generally applicable to all DirectQuery sources and which aren’t in the guidance docs (yet), such as the importance of setting the Maximum Connections Per Data Source property (which I also blogged about recently here) and the fact you can increase this to 30 in a Premium capacity, as well as the importance of always testing DirectQuery performance in the Power BI Service rather than in Power BI Desktop. As a result it’s worth reading if you are thinking of using Power BI in DirectQuery mode with Synapse, BigQuery or any other source.
If you are considering using DirectQuery on a project I have one last piece of advice: think very carefully about why you need to use DirectQuery and not Import mode. Many people don’t and end up in trouble – in my opinion Import mode should be the default choice simply because it will almost always perform better than DirectQuery mode, whatever back-end database you’re using.
After last week’s post on measuring Power Query CPU usage during dataset refresh, someone asked an obvious question that I should have addressed: does using a gateway change anything? After all, if you’re using a gateway to connect to an on-premises data source then all the Power Query queries transforming the data from that source will be executed on the gateway machine and not in the Power BI Service.
Let’s do a quick test to find out. I couldn’t use the same Power Query query I used in last week’s post (it turns out you can’t force the use of a gateway when there isn’t an external data source) so instead I used another dataset that connects to a large CSV stored in ADLSgen2 storage and does a group by operation – something which is guaranteed to be very expensive in terms of CPU for Power Query.
Here’s what Profiler shows for the refresh operation when no gateway is used:
The refresh took around 30 seconds and used around 44 seconds of CPU time.
Here’s what Profiler shows when the refresh does use a gateway:
The refresh takes a lot longer, around 103 seconds (as you would expect – instead of loading the data from ADLSgen2 storage in the cloud to the Power BI Service, it has to take a round trip via the gateway on my PC) but the important thing is that the CPU time is now very low – 141 milliseconds.
So, as you might expect, the CPU time for refreshes that use an on-premises data gateway is not shown in Profiler traces because, as I said, all the work done by the Power Query engine is done on the gateway machine and not in the Power BI Service. Making refreshes use a gateway, even when you don’t need to, can be a way of taking load off a Power BI Premium capacity if it’s overloaded.
This in turn raises the question of how you measure Power Query CPU usage on a gateway? As far as I know it isn’t possible for individual Power Query queries (I could be wrong though), although the gateway logs do allow you to capture CPU usage for the whole machine. Better gateway monitoring tools are on the way but this seems like a good time to mention my colleague Rui Romano’s open source gateway monitoring solution (article | repo) which makes understanding the gateway logs a lot easier.
Some time ago I wrote a post about how optimising for CPU Time is almost as important as optimising for Duration in Power BI, especially if you’re working with Power BI Premium Gen2. This is fairly straightforward if you’re optimising DAX queries or optimising Analysis Services engine-related activity for refreshes. But what about Power Query-related activity? You may have a small dataset but if you’re doing a lot of complex transformations in Power Query that could end up using a lot of CPU, even once the CPU smoothing for background activity that happens with Premium Gen2 has happened. How can you measure how expensive your Power Query queries are in terms of CPU? In this post I’ll show you how.
Let’s consider two Power Query queries that return a similar result and which are connected to two different tables in the same Power BI dataset. The first query returns a table with one column and one row, where the only value is a random number returned by the Number.Random M function:
#table(type table [A=number],{{Number.Random()}})
The second query also returns a table with a single value in it:
let
InitialList = {1 .. 1000000},
RandomNumbers = List.Transform(
InitialList,
each Number.Random()
),
FindMin = List.Min(RandomNumbers),
Output = #table(
type table [A = number],
{{FindMin}}
)
in
Output
This second query, however, generates one million random numbers, finds the minimum and returns that value – which of course is a lot slower and more expensive in terms of CPU.
If you run a SQL Server Profiler trace connected to Power BI Desktop and refresh each of the two tables in the dataset separately, the Command End event for the refresh will tell you the duration of the refresh and also the amount of CPU Time used by the Analysis Services engine for the refresh (there will be several Command End events visible in Profiler but only one with any significant activity, so it will be easy to spot the right one). In Desktop, however, the Command End event does not include any CPU used by the Power Query Engine. Here’s what the Command End event for the first Power Query query above looks like in Desktop:
As you would expect the values in both the Duration and CPU Time columns are low. Here is what the Command End event looks like for the second query above:
This time the refresh is much slower (the Duration value is much larger than before) but the CPU Time value is still low, because the Analysis Services engine is still only receiving a table with a single value in it. All the time taken by the refresh is taken in the Power Query engine.
If you publish a dataset containing these queries to a Premium workspace in the Power BI Service, connect Profiler to the XMLA Endpoint for the workspace, and then refresh the two tables again then for the first, fast query you won’t notice much difference:
[Note that in this screenshot I’ve chosen a comparable Command End event to the one I used in Desktop, although for some reason it doesn’t show the duration. The overall refresh duration, which includes some extra work to do a backup, is around 2 seconds]
However, for the second, slower query you can see that the CPU Time for the Command End event is much higher. This is because in the Power BI Service the event’s CPU Time includes all the Power Query-related activity as well as all Analysis Services engine activity:
This is a simple example where there is very little work being done in the Analysis Services engine, which means that pretty much all the CPU Time can be attributed to the Power Query engine. In the real world, when you’re working with large amount of data, it will be harder to understand how much work is being done in the Analysis Services engine and how much is being done in the Power Query engine. This is where Power BI Desktop comes in, I think. In Desktop you know you are only seeing the CPU used by the Analysis Services engine, so I’ll bet that if there is a big difference in the ratio of CPU Time to Duration for your refresh in Power BI Desktop compared to the Power BI Service, it’s highly likely that that difference is due to Power Query engine activity and that’s where you should concentrate your optimisation efforts.
Of course the next question is how can you optimise Power Query queries so they use less CPU? I don’t know, I haven’t done it yet – but when I have something useful to share I’ll blog about it…
Power Query Online is, as the name suggests, the online version of Power Query – it’s what you use when you’re developing Power BI Dataflows for example. Sometimes when you’re building a complex, slow query in the Query Editor you’ll notice a message in the status bar at the bottom of the page telling you how long the query has been running for and how much memory and CPU it’s using:
The duration and CPU values are straightforward, but what does the memory value actually represent? It turns out it’s the “Commit (Bytes)” value documented here for Query Diagnostics, that’s to say the amount of virtual memory being used by the query. That’s different to the “Working Set (Bytes)” value which is the amount of physical memory used by the query, and which is not visible anywhere. For a more detailed discussion of these values in Power Query in Power BI Desktop see this post. The maximum commit or working set for a query evalation in Power Query Online isn’t officially documented anywhere (and may change) but I can say three things:
The maximum commit is larger than the maximimum working set.
If Power Query Online uses more than the maximum working set then query evaluation will get slow, so if your query uses a lot of memory (say, over 1GB – I suspect you’ll only see this message if it is using a lot of memory…) then you need to do some tuning to reduce it. Probably the best way to do this is to look at the query plan for your dataflow and try to avoid any operations marked as “Full Scan”, as documented here.
If your query uses more than the maximum commit then it may get cancelled and you’ll see an error (note that the maximum time a query evaluation can run for in Power Query Online anyway is 10 minutes, which is documented here).
[Thanks to Jorge Gomez Basanta for this information]
The most exciting (at least for me) feature in the new Enhanced Refresh API (blog announcement | docs) is the ability to cancel a dataset refresh that’s currently in progress. Up until now, as this blog post by my colleague Michael Kovalsky shows, this has been quite difficult to do: not only do you need to use the XMLA Endpoint but you also need to take into account that in many cases the Power BI Service will automatically restart the refresh even after you’ve cancelled it. Now, though, if (and only if) you start the refresh using the Enhanced Refresh API you can also cancel it via the Enhanced Refresh API too. This is important because I’ve seen a few cases where rogue refreshes have consumed a lot of CPU on a Premium capacity and caused throttling, even after all CPU smoothing has taken place, and Power BI admins have struggled to cancel the refreshes.
This and all the other great functionality the new API includes (the ability to refresh individual tables or partitions! control over parallelism!) means that it can handle many of the advanced scenarios that, in the past, you’d have had to write some complex TMSL commands for; in my opinion anyone working on an enterprise-level dataset in Power BI Premium should be using it for their refreshes.
But Chris, I hear you say, I’m a data person and find working with APIs confusing and difficult! Yeah, me too – which is why, when I saw this tweet by Stephen Maguire about .NET interactive notebook for Visual Studio Code he’s built for the Enhanced Refresh API, I was interested:
It’s a really great set of examples for learning how to use the Enhanced Refresh API through PowerShell and the notebook format makes it a lot more user-friendly than just another bunch of scripts. I highly recommend that you check it out.
If you’re working with DirectQuery in Power BI then one of the most important properties you can set on your dataset is the “Maximum connections per data source” property. You can find it on the Published Dataset Settings tab in the Options dialog in Power BI Desktop:
You can set the maximum number of connections DirectQuery opens for each underlying data source. It controls the number of queries concurrently sent to the data source.
The setting is only enabled when there’s at least one DirectQuery source in the model. The value applies to all DirectQuery sources, and to any new DirectQuery sources added to the model.
Increasing the Maximum Connections per Data Source value ensures more queries (up to the maximum number specified) can be sent to the underlying data source, which is useful when numerous visuals are on a single page, or many users access a report at the same time. Once the maximum number of connections is reached, further queries are queued until a connection becomes available. Increasing this limit does result in more load on the underlying data source, so the setting isn’t guaranteed to improve overall performance.
When the model is published to Power BI, the maximum number of concurrent queries sent to the underlying data source also depends on the environment. Different environments (such as Power BI, Power BI Premium, or Power BI Report Server) each can impose different throughput constraints.
I thought it would be interesting to do some experiments to see how this property behaves, what you see in Profiler (or Log Analytics) when connections are queued up, and how you can find an optimal value for your dataset.
The first thing to mention – and this is something I only realised relatively recently – is that this property applies to DirectQuery on Power BI datasets and Analysis Services as well as traditional DirectQuery to external databases. I’m a lot more comfortable with Power BI than any relational database so I decided to do my testing with a DirectQuery dataset connected back to another Power BI dataset; the behaviour of the feature is the same as with DirectQuery to a relational database.
For my tests I created a simple dataset – let’s call it Dataset A – with not much data but a really inefficient DAX measure on it. I then created a composite model dataset – let’s call this Dataset B – with a DirectQuery connection to Dataset A. Finally I created a report with a Live connection to Dataset B with 25 card visuals on, each of which used the inefficient measure with a different filter. The DAX query for each of these cards, when run on its own through DAX Studio, took around 28 seconds, with almost all that time spent in the Formula Engine. The datasets and reports were published to a PPU workspace and all tests were run in the Power BI Service and not in Power BI Desktop (at the time of writing, things work differently in Desktop – which means you should always test performance of DirectQuery reports in the Service and not in Desktop). I ran Profiler traces using the Query Begin and Query End events on both Dataset A and Dataset B during my tests.
First of all, let’s see what happened when the Maximum Number of Connections property on Dataset B was set to 1. This means that Dataset B is only allowed to have one connection open to Dataset A to run its DirectQuery queries. When the report was run, right at the start the Profiler trace on Dataset B showed 25 Query Begin events indicating all 25 queries for the 25 card visuals were being run in parallel; the Profiler trace on Dataset A showed just 1 Query Begin event:
This is what you would expect: since Dataset B can only use one connection to Dataset A it can only run one query at a time and the other 24 queries have to queue up to wait for the connection. When the first query against Dataset A completed, another one started and so on. Since the maximum length of time that a DAX query can run in the Power BI Service is 225 seconds, after 225 seconds any remaining queries timed out. At that point 8 queries had completed, so 8 cards were rendered, and all the remaining cards showed the timeout error I blogged about here:
At the end, the Profiler trace against Dataset A showed 8 completed Query Begin/End pairs:
While the Profiler Trace against Dataset B showed, after the 25 Query Begin events, 8 Query End events for the successful queries and then 17 Query End events with timeout errors for the unsuccessful queries.
One interesting thing to notice is the durations of the queries. As I said, when run on their own each of these queries took around 28 seconds, and the Profiler trace on Dataset A shows each query taking around 28 seconds. If you look at the successful queries on Dataset B you’ll see that their duration goes up in increments of around 28 seconds: the first takes 29984ms, the second takes 58191ms, the third takes 87236ms and so on until you hit the 225 second timeout limit. This shows that the duration of the queries against Dataset B, the composite model, includes the time waiting to acquire a connection. Notice also that the CPU Time of the queries against Dataset B is minimal because it only includes the CPU used by the query for Dataset B; you have to add the CPU Time to the related queries on Dataset A to get the total CPU Time used by these queries.
The important question is, though, what is the effect of increasing the Maximum Connections Per Data Source property? Increasing it will increase the number of queries run in parallel, but is more paralellism always better? I reran my tests with the property set to 5, 10 (which is the default value and the maximum that can be used for datasets not in Premium capacity) and 30 (which is the maximum value that can be used for datasets in any form of Premium capacity). Here are the results:
Maximum Connections Per Data Source
Number of Visuals That Render Successfully In 225 Seconds
1
8
5
16
10
10
30
8
As you can see increasing the parallelism a little bit helps more than increasing the parallelism a lot, and in this case reducing the value from the default was better than increasing it: overloading your source with a lot of expensive parallel queries is often a bad thing. This test isn’t representative of most real-world reports – you shouldn’t have one visual, let alone 30, with queries that run for as long as 30 seconds and the best way to optimise a report like this would be to display the same data in a smaller number of visuals – but I think it’s a useful illustration of how this property works and how it can affect report performance.
In amongst all the announcements at Build recently, you may have heard about a new member of the Power Platform being launched: Power Pages. You can read the docs here, and there’s a good, detailed video overview here, but here’s a quick summary of what it is:
Microsoft Power Pages is a secure, enterprise-grade, low-code software as a service (SaaS) platform for creating, hosting, and administering modern external-facing business websites. Whether you’re a low-code maker or a professional developer, Power Pages enables you to rapidly design, configure, and publish websites that seamlessly work across web browsers and devices.
So what? I’m not a web designer and I’m pretty sure most of you aren’t either, so why blog about it here? Most data and BI people don’t need to build web sites… do they?
Well I was playing around with it and noticed one important detail:
It has built-in support for embedding Power BI reports into the web sites you build! You can read more about this here and here. What’s more, it supports all forms of Power BI embedding (which can be an extremely confusing subject): as well as the use of Power BI Embedded, for sharing reports with external users, you can use regular Power BI Premium, Secure Embedding (which doesn’t need Premium), and Publish to Web for sharing with the general public. It also supports embedding of reports and dashboards (though not paginated reports) and also more complex security scenarios if you have the relevant web development skills.
As someone with no web development skills whatsoever it was very easy for me to build a web site with a Power BI report embedded into it:
When would this be useful? I talk to a lot of people who want to share Power BI reports with external users. You can use Azure B2B for this, although it doesn’t give you the smooth experience a custom-built web site does – but using a custom-built web site of course requires you to actually build that web site, and not everyone has a web developer available to do this work. This is where I see Power Pages being extremely useful: self-service web development for self-service data people, letting you share data securely outside your organisation quickly and easily.
There’s an old joke about “Export to Excel” being the most important feature of any BI tool. In fact, I’d say export to Excel is one of the most important features of any enterprise application of any type. Of course the reason we joke about it is that we know it’s a Bad Thing and the starting point for all kinds of manual, error-prone and time-consuming business processes – but even though we know there are much better ways of achieving whatever it is the user wants to do, they still want to export to Excel.
So wouldn’t be good if you could export to Excel and instead of getting a static copy of the data, you could get a table connected to a Power Query query which in turn connected back to the original data source, so it could be refreshed whenever the user wanted? After all, pretty much everyone nowadays has a version of Excel with Power Query in it (even, with some limitations, Mac users). It’s always been possible to build yourself but technically difficult. Recently, though, I became aware of a JavaScript library developed by the Excel Power Query team called “Connected Workbooks” that makes it extremely easy to do this. You can find out more about it here:
