Query folding is when steps defined in Power Query or Query Editor are translated into SQL and executed by the source database rather than the client machine. It is important to processing performance and scalability, given limited resources on the client machine.
In Power Query, query folding refers to the optimization technique used to improve the performance of data transformation operations. When you connect to a data source in Power Query and apply transformations, Power Query attempts to push down those transformations to the data source itself rather than loading all the data into memory and then performing the transformations.
By utilizing query folding, Power Query generates and sends optimized queries to the data source, allowing it to perform the transformations directly at the source. This reduces the amount of data transferred between the data source and Power Query, leading to faster and more efficient data processing.
In an interview, you could explain that query folding is a crucial aspect of Power Query for optimizing data retrieval and transformation operations, resulting in improved performance and scalability, especially when dealing with large datasets or complex transformations. Additionally, you could mention that not all transformations can be folded, as it depends on the capabilities of the data source and the specific transformations being applied.