Which Power Query transformation generally prevents query folding and should be avoided if you want to preserve folding?

• A. Adding index columns
• B. Keep rows
• C. Pivot
• D. Unpivot

Answer: (A)

Query folding happens when Power Query translates as many steps as possible into the data source’s native query language, letting the source do the work. Adding an index column generally cannot be represented by the source in a SQL-like query, so this step stops folding. When folding is lost, subsequent transformations run locally after the data is loaded, which can slow things down because more data is retrieved and processed on the client. To keep folding, avoid adding an index early; if you need an index, implement it in the source or add it after folding-friendly steps are completed.