Power Query

What is query folding?

Power Query translating steps into a single native query the source executes, instead of transforming locally.

How do you check if folding is happening?

Right-click a step and look for View Native Query. Greyed out means folding stopped.

Name a common folding breaker.

Adding an index column; also some custom M functions and merges with non-foldable sources.

Unpivot — when do you use it?

To turn wide data (one column per month, say) into tall attribute/value rows the model can use.

Merge vs Append?

Merge is a join (adds columns); Append is a union (adds rows).

Is M case sensitive?

Yes. Table.SelectRows works; table.selectrows does not.

What does try ... otherwise do?

Catches a step error and substitutes a fallback value, preventing one bad row failing the refresh.

Where should you remove unused columns?

In Power Query, as early as possible, so they never enter the model.