What is query folding in Power Query ? Power BI interview questions and answers 094

  What is query folding in Power Query ?

Query folding in Power Query is a performance optimization technique that pushes data transformation steps you define in Power Query back to the data source whenever possible. This means that instead of Power Query loading all the data into its own engine and performing transformations locally, it leverages the processing power of the data source itself, often leading to significantly faster query execution and reduced memory usage.

Here's how query folding works:

1. You apply transformations: Within Power Query Editor, you define steps such as filtering, sorting, grouping, joining, or aggregating data.

2. Query analysis: Power Query analyzes those steps to determine if they can be translated into equivalent operations that the data source itself can understand and execute.

3. Pushing down steps: If possible, Power Query generates a query in the native language of the data source (e.g., SQL for relational databases, M for Excel) and sends it directly to the source for execution.

4. Source execution: The data source performs the requested transformations on its own, returning only the filtered, sorted, or aggregated results to Power Query.

5. Result shaping: Power Query receives the processed data and performs any remaining transformations that couldn't be folded, ensuring the final output matches your desired format.

Benefits of query folding:

• Significant performance improvements: Large datasets can be processed much faster, especially when dealing with millions of rows or complex transformations.

• Reduced memory usage: By offloading work to the data source, less data needs to be loaded into Power Query's memory, conserving resources for other tasks.

• Optimized data source utilization: The data source's processing capabilities are fully leveraged, potentially leading to better efficiency than Power Query's own engine.

Factors affecting query folding:

• Data source support: Not all data sources support query folding to the same extent. Relational databases like SQL Server, Azure SQL Database, and some cloud-based services generally have excellent folding capabilities.

• Transformation type: Some transformations, like custom functions or conditional logic, might not be foldable, requiring local execution within Power Query.

• Data connectivity: Direct connection modes have a higher chance of query folding compared to import modes.

To maximize query folding:

• Utilize supported data sources.

• Favor direct connections whenever possible.

• Be mindful of transformation types and their foldability.

• Monitor query execution in Power Query's diagnostic tools to see which steps are folded and which are executed locally.

Query folding is a powerful feature that can significantly boost the performance of your Power Query data transformations. By understanding its mechanisms and best practices, you can ensure efficient data processing and leverage the strengths of both Power Query and your data sources for optimal results.