If the title wasn’t enough to warn you, this post is only going to be of interest to M ultra-geeks and people using Power BI with Azure Data Explorer – and I know there aren’t many people in either group. However I thought the feature I’m going to show you in this post is so cool I couldn’t resist blogging about it.
Let’s say I have a table in Azure Data Explorer called Product that looks like this:
Because this table is in Azure Data Explorer I can query it with KQL, and the thing I love most about KQL as a language is the way that each query is built up as a series of piped transformations. So, for example, on the table above I could write the following query:
Product | where Type=='Berry' | where FruitID >1
…and get this result:
It seems quite similar to the way M works, right? Anyway, now for the cool part. When you connect Power Query in Power BI up to Azure Data Explorer, not only does query folding take place but you can mix and match native KQL fragments (added using the Value.NativeQuery M function) and Power Query transformations created in M, and it all still folds! This completely blew my mind when I found out about it. It’s documented briefly here but the docs don’t explain everything that you can do with it.
Consider the following M query:
let Source = AzureDataExplorer.Contents( "https://xyz.ukwest.kusto.windows.net/", "MyDatabase", "Product" ), #"Filtered Rows" = Table.SelectRows( Source, each ([Type] = "Berry") ), Q1 = Value.NativeQuery( #"Filtered Rows", "| where FruitID>2" ) in Q1
There are three steps:
- Source connects to the Product table
- #”Filtered Rows” is a filter generated by the Power Query UI to filter the table down to just the rows where Type is “Berry”
- Q1 uses Value.NativeQuery to add a KQL filter onto the output of #”Filtered Rows” so only the rows where FruitID is greater than 2
Here’s the KQL this folds to:
Product | where strcmp(["Type"], ("Berry")) == 0 | where FruitID>2
You can also use Value.NativeQuery more than once. Here’s another M query that returns the same result as above:
let Source = AzureDataExplorer.Contents( "https://xyz.ukwest.kusto.windows.net/", "MyDatabase", "Product" ), Q3 = Value.NativeQuery( Source, "| where Type=='Berry'" ), Q4 = Value.NativeQuery( Q3, "| where FruitID>2" ) in Q4
Very useful when you want to build a query quickly using the Power Query Editor but then need to use a KQL feature that isn’t supported by Power Query.