IF, SWITCH And The Effect Of DAX Variables On Strict/Eager Evaluation

A lot of DAX performance problems relate to the use of strict and eager evaluation with the IF or SWITCH functions. It’s an incredibly complex, “it depends”, black-box type of topic and naturally Marco and Alberto have a great article on it here. Rather than describe any general rules – which would be pretty much impossible – in this blog post I want to show a specific scenario that illustrates how the use of DAX variables can influence whether strict or eager evaluation takes place.

Consider a Power BI semantic model that has the following three tables:

TableA and TableB are fact tables with numeric measure columns; Choice is a disconnected table containing text values that is intended for use in a slicer, so a report user can select an item in that slicer and that in turn influences what values a measure returns:

Here’s the definition of one such measure:

With Variables =
VAR a =
    SUM ( TableA[A] )
VAR b =
    SUM ( TableB[B] )
RETURN
    IF ( SELECTEDVALUE ( Choice[Choice] ) = "TableA", a, b )
)

And here’s my test report that includes a slicer and a card visual displaying the output of this measure:

Let’s look at the DAX query generated by the card visual containing the measure when “TableA” is selected in the slicer:

DEFINE
    VAR __DS0FilterTable =
        TREATAS ( { "TableA" }, 'Choice'[Choice] )

EVALUATE
SUMMARIZECOLUMNS (
    __DS0FilterTable,
    "With_Variables", IGNORE ( 'Choice'[With Variables] )
)

…and in particular what DAX Studio’s Server Timings feature shows us:

Even though only TableA is selected in the slicer and the query only returns the sum of the values in the A column of TableA, we can see that the Storage Engine is also querying TableB and getting the sum of the B column. It’s a great example of eager evaluation: both branches of the IF are being evaluated. Is this a bad thing? For this particular report it may be if the Storage Engine query for TableB is expensive.

How can you force strict evaluation to take place? You can force eager evaluation using the IF.EAGER function but there is no equivalent function to force strict evaluation. However you maybe be able to rewrite the measure to get strict evaluation to take place.

The key factor in this case is the use of variables in the measure definition. If you rewrite the measure to not use variables, like so:

No Variables =
IF (
    SELECTEDVALUE ( Choice[Choice] ) = "TableA",
    SUM ( TableA[A] ),
    SUM ( TableB[B] )
)

…then the query for the card visual query behaves differently:

DEFINE VAR __DS0FilterTable = 
	TREATAS({"TableA"}, 'Choice'[Choice])

EVALUATE
	SUMMARIZECOLUMNS(__DS0FilterTable, "No_Variables", IGNORE('Choice'[No Variables]))

Notice that there are now only two Storage Engine queries and that TableB is not now being queried, which will probably result in better performance. This is strict evaluation.

Why does the use of variables result in the use of eager evaluation here? Because it does, and it’s complicated. I need to stress that DAX uses lazy evaluation for variables which means that variables are not evaluated if they are not used – in the first measure above the IF is deliberately evaluating both branches. There are certainly other optimisations that may kick in and result in strict evaluation even when variables are used in IF/SWITCH. Indeed, the two measures in this example being from different fact tables is extremely important: if they had been from the same fact table then the behaviour would have been different and strict evaluation would have been used. In summary, though, if you are using variables and you want to try to force strict evaluation with IF/SWITCH then it’s worth rewriting your code to remove those variables to see if it makes a difference.

I also need to stress that these two measures will perform better or worse depending on how and where they are used. Consider these two table visuals that use the values from the Choice table on rows along with the two measures above:

Running the DAX query for the table on the left, which uses the measure with no variables and strict evaluation, gives the following in DAX Studio’s Server Timings:

Note that there are two Storage Engine queries for TableB, which is not a good thing.

On the other hand, the table on the right which uses the [No Variables] measure gives the following:

Note that there is only one Storage Engine query for TableB now, so in this case the tables have turned and the [With Variables] measure is likely to perform better.

When you use variables in the branches of IF/SWITCH, as in the [With Variables] measure, the variables are evaluated in a context at the place of the variable definition; if you don’t use variables in this way, as in the [No Variables] measure, the two SUM expressions used in the two branches are evaluated in the context of the branch, which adds a hidden filter context corresponding to the condition. This has two consequences which may hurt query performance:

1. During evaluation, the hidden filter may be materialised into a large in-memory table
2. Fusion cannot happen across common subexpressions in different branches because they have different contexts

In contrast the use of variables means the expressions used in them are evaluated in a context without the hidden filter corresponding to the branch, which can encourage fusion and discourage the materialisation of large in-memory tables.

[Thanks to Marius Dumitru and Jeffrey Wang for much of the information in this post]