M Query Folding

What is M Query Folding?

M Query Folding is a feature in Microsoft's Power Query that enhances performance by translating transformations into native database queries. This process allows complex data transformations to be executed directly on the data source, minimizing the amount of data transferred and processed in Power Query.

How M Query Folding Works

M Query Folding takes the transformations defined in a Power Query M script and attempts to convert them into a single SQL statement that the source database can understand and process. This reduces the workload on the client side and leverages the database engine's capabilities to handle large datasets more efficiently.

Benefits of Query Folding

• Reduces data transfer costs and times by executing transformations within the database.
• Utilizes the optimized performance of the database engine.
• Improves overall performance of data retrieval and transformation processes.

Examples of Query Folding

Consider a scenario where you want to filter and aggregate data from a SQL Server database. With M Query Folding, these operations can be pushed down to the SQL Server, minimizing data movement.

Best Practices for Maximizing Query Folding

To maximize the effectiveness of query folding, consider the following practices:

• Use built-in Power Query functions that are more likely to support folding.
• Avoid transformations that cannot be folded, such as custom functions or operations performed after data has been loaded.
• Enable query diagnostics to monitor and confirm which steps are being folded.

Limitations of M Query Folding

While M Query Folding can significantly enhance performance, it has some limitations:

• Not all transformations can be folded, especially those that require data to be pulled into memory.
• Some data sources may have limited support for query folding.
• The complexity of the M query might prevent folding if the database cannot represent the transformations in SQL.