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DirectQuery Partitions on Hybrid Tables

Shabnam Watson noticed something interesting:

While I was tracing some Power BI datasets with Hybrid tables, I noticed that that the DirectQuery partition of a Hybrid table is used in all queries submitted to the Hybrid table, whether or not they are asking for data from that Direct Query partition. On first look, this does not look good because Power BI should be able to use partition pruning for a partitioned table, meaning it should only scan certain partitions. For example, if you ask for historical data from 5 years ago, the current day’s data is not needed to answer that query and hence, the Direct Query partition for today should not be used in the query.

But do read on for more information about what’s going on, as it seems there are layers of nuance here.