A quick update, as tonight I managed to sit down and experiment a bit with the V2 Beta for the first time in many days.

(OK, actually, I am standing up…  because I have a fantastic new treadmill desk and have walked nearly 8 miles today while working, but that’s a topic for another post).

I was working with one of our HostedPowerPivot clients today who was observing slower-than-expected slicer click performance in one of their reports.  And after some sleuthing, I found that they had a “distinct count” measure in their model, something like this:

[Measure] = COUNTROWS(DISTINCT(Table[Column]))

Now, that’s a pretty useful measure in many situations, like “does product X sell every day?”  Simply taking a count of the date column doesn’t cut it, because if it sells twice on one day and not at all the next day, the count is still 2, and you want it to be 1.

Trouble is, in PowerPivot v1, that operation can be slow.  I won’t bore you with all the details, because frankly, I don’t know all of them, heh heh.  But I do know that PowerPivot v1 does end up creating all kinds of temporary new tables in memory behind the scenes when it is evaluating the measure.

Anyway, I’d heard a long time ago that PowerPivot V2 was going to make distinct counts a lot faster, so I thought I’d try it out.

The Test Case

First I needed to create a test case that was challenging.  I don’t want to compare “fast query vs. fast query” because that often gets skewed by fixed overhead.  Much better to test a “worst case scenario.”

So, I took a 15 million row table, and added a calculated column to it.  Calculated columns in PowerPivot are much less compressed than imported columns, and are therefore more challenging for the measure engine to scan through, looking for duplicates etc. which is required for a distinct count.

And then, to make matters worse, I just used the RAND() function so that there are many unique values:

image

Cutting to the Chase:  The Results

A distinct count measure in PowerPivot v1 took 35 seconds to complete in my test pivot.  This was a big part of why our client was seeing slow perf today.

That same measure, in that same pivot, took less than a second in the V2 beta.

Wow.  This is gonna be nice.

One Last Note:  Two Ways to Write the Formula

PowerPivot v1 did not have a dedicated function for distinct count, you had to do the countrows of distinct thing like what I showed above.

But in PowerPivot v2, there IS a DISTINCTCOUNT() function.  So you can write your distinct count measures in two ways:

[Measure] = COUNTROWS(DISTINCT(Table[Column]))

[Measure] = DISTINCTCOUNT(Table[Column])

That new function is nice, makes for a more readable formula.  BUT…  the performance is still the same.  Either way I wrote it, the measure was sub-second fast.  Very nice.