What is query collapsing in Power BI ?Power BI interview questions and answers 095

 What is query collapsing in Power BI ?

While "query collapsing" isn't a standard term within Power BI, it's likely used to describe a technique that aims to streamline query execution and improve performance. It involves combining multiple Power Query steps into a single step whenever possible, reducing the number of times data needs to be loaded and processed.

Here's how it works:

1. Analyze Steps: Power BI analyzes the sequence of transformations in your query to identify opportunities for collapsing.

2. Combine Steps: If multiple steps can be logically combined into a single operation without affecting the output, they are merged into one step.

3. Reduce Data Loading: This consolidation minimizes the number of times data needs to be loaded or processed, enhancing efficiency.

Benefits of query collapsing (or step consolidation):

• Faster query execution: By reducing the number of steps, data processing time is often shortened.

• Reduced memory usage: Fewer steps often mean less data needs to be held in memory during processing, conserving resources.

• Improved query readability: A simpler query structure with fewer steps can be easier to understand and maintain.

Common techniques for collapsing queries:

• Combining filtering and sorting: If you're filtering and then sorting based on the same column, Power BI might collapse those steps into a single operation.

• Grouping and aggregating: When grouping and then performing calculations on the grouped data, Power BI might combine those steps to optimize processing.

• Removing unnecessary steps: Eliminating steps that don't alter the data (e.g., renaming a column and then renaming it back) can also streamline the query.

Points to remember:

• Query folding takes precedence: If a set of steps can be folded to the data source, that's generally preferred over collapsing them within Power BI.

• Not always possible: Collapsing isn't always feasible for all combinations of steps, as it depends on the specific operations involved.

To facilitate query collapsing:

• Structure your queries thoughtfully: Consider the order of steps and potential consolidation opportunities.

• Use the "View Native Query" option: Examine the generated SQL to see how Power BI has optimized the query and identify any remaining areas for improvement.

While not a formal term, query collapsing (or step consolidation) represents an important concept for optimizing Power BI query performance. By understanding its mechanisms and applying best practices, you can create more efficient and performant data transformations.