Azure Data Explorer (ADX) is a great platform for storing large amounts of transactional data. The Incremental Refresh feature (now available for Pro users!) in Power BI makes it much faster to keep data models based on that data current. Unfortunately, if you follow the standard guidance from Microsoft for configuring Incremental Refresh, you’ll quickly bump into a roadblock. Luckily, it’s not that difficult to get around.
Incremental Refresh works by setting up data partitions in the dataset in the service. These partitions are based on time slices. Once data has been loaded into the dataset, only the data in the most recent partition is refreshed.
To set this up in Power BI Desktop, you need to configure two parameters, RangeStart, and RangeEnd. These two parameters must be set as Date/Time parameters. Once set, the parameters are used to filter the Date/Time columns in your tables accordingly, and once published to the service, to define the partitions to load the data into.
When Power Query connects to ADX, all Date/Time fields come in as the Date/Time/Timezone type. This is a bit of a problem. When you use the column filters to filter your dates, the two range parameters won’t show up because they are of a different type (Date/Time). Well, that’s not a big problem, right? Power Query lets us change the data column type simply by selecting the type picker on the column header.
Indeed, doing this does in fact allow you to use your range parameters in the column filters. Unfortunately, data type conversions don’t get folded back to the source ADX query. You can see this by right-clicking on a subsequent step in the Power Query editor. The “View Native Query” option is greyed out, which indicates that the query cannot be folded.
Query folding is critical to incremental refresh. Without it, the entirety of the data is brought locally so that it can be filtered vs having the filter occur at the data source. This would completely defeat the purpose of implementing Incremental Refresh in the first place.
The good news is that you can in fact filter a Date/Time/Timezone column with a Date/Time parameter, but the Power Query user interface doesn’t know that. The solution is to simply remove the type conversion Power Query step AFTER performing the filter in the Power Query UI.
Alternatively, if you’re comfortable with the M language, you can simply insert something like the following line using the Advanced Editor in Power Query (where CreatedLocal is the name of the column being filtered).
#"Filtered Rows" = Table.SelectRows(Source, each [CreatedLocal] >= RangeStart and [CreatedLocal] < RangeEnd),
If the filtration step can be folded back into the source, Incremental Refresh should work properly. You can continue setting up Incremental Refresh using the DAX editor. You will likely see some warning messages indicating that folding can’t be detected, but these can safely be ignored.