Value.ResourceExpression() And Query Folding In Power BI

When the June update for Power BI Desktop was released, I was extremely excited to discover an undocumented new feature that allows you to see when query folding is taking place while loading data in the Query Editor. Devin Knight beat me to blogging about it with an excellent post that I strongly recommend you to read:

https://devinknightsql.com/2016/07/03/power-bi-checking-query-folding-with-view-native-query/

As Devin says, this is very important because it’s the first time that we as developers can check to see whether query folding is taking place within the Power BI Query Editor (I guess this feature will come to Power Query soon, maybe sometime in autumn 2016), and getting query folding working is the key to getting good performance when loading data.

Looking at this more closely, I’ve found that it’s also possible to get the query generated in code as well as just seeing it in a dialog box. For example, consider the following M code that returns a few columns and some filtered rows from the DimDate table in the Adventure Works DW SQL Server database:

let
    Source = Sql.Database("localhost", "adventure works dw"),
    dbo_DimDate = Source{[Schema="dbo",Item="DimDate"]}[Data],
    #"Removed Other Columns" =
	Table.SelectColumns(
		dbo_DimDate,
		{"DateKey", "FullDateAlternateKey",
		"DayNumberOfWeek"}),
    #"Filtered Rows" =
	Table.SelectRows(
		#"Removed Other Columns",
		each ([DayNumberOfWeek] = 1))
in
    #"Filtered Rows"

You can confirm that query folding is taking place and see the SQL that is being generated for the Filtered Rows step by right-clicking on it in the Applied Steps pane and choosing View Native Query:

If you now add two lines of code to the query you can get the SQL query for the #”Filtered Rows” step as a text value using the Value.ResourceExpression() function, like so:

let
    Source = Sql.Database("localhost", "adventure works dw"),
    dbo_DimDate = Source{[Schema="dbo",Item="DimDate"]}[Data],
    #"Removed Other Columns" =
	Table.SelectColumns(
		dbo_DimDate,
		{"DateKey", "FullDateAlternateKey",
		"DayNumberOfWeek"}),
    #"Filtered Rows" =
	Table.SelectRows(
		#"Removed Other Columns",
		each ([DayNumberOfWeek] = 1)),
    FindQueryRecord =
        Value.ResourceExpression(#"Filtered Rows"),
    Output =
        FindQueryRecord[Arguments]{1}[Value]
in
    Output

As far as I can see, the Value.ResourceExpression() function can be used with any expression where query folding is taking place and returns a record which, if you dig deep enough into it contains the SQL query. The record itself seems to contain all the information needed to invoke the new Value.NativeQuery() function which I’ve also been looking at recently – but I won’t be blogging about just yet because I’m told it’s not quite finished.

Why is it useful to get the SQL query as text, rather than just seeing it? Well… I have some ideas, and hopefully some other upcoming blog posts will make things clearer!