During one of my presentations on Incremental Refresh (IR) in Power BI, someone asked what happens during a Power BI automatic partition maintenance window when Power BI has an opportunity to merge smaller partitions into larger ones. Does Power BI use the data that is already imported into Power BI for the smaller partitions and combine it into a bigger one or does it re-read the data for those smaller partitions again. For example, if a dataset has an IR policy to refresh the last 1 day, and it has read data for all the days in a previous month, one day for each, when the new month arrives, it has an opportunity to merge the smaller day partitions into a month partition for the previous month. Does it re-read the previous month’s data from the source again or does it use what it already has in its memory?
Read on for the answer.