Power Query (M Language)

Overview

Power Query is the data transformation engine in Power BI, using the M functional language. It handles ETL (Extract, Transform, Load) from sources to the data model. Understanding query folding, step optimization, and M syntax is critical for performant data refresh.

Query Folding

Query folding translates M steps into native source queries (SQL, OData, etc.), pushing computation to the source instead of the mashup engine.

How to check folding:

1. Right-click a step in Applied Steps > "View Native Query" -- if grayed out, folding broke
2. Use Query Diagnostics (Tools > Start Diagnostics) to see what queries are sent