In Pentaho Data Integration (PDI), when uniting two or more data flows into a single step, the streams are effectively merged. A fundamental prerequisite for this operation is that the metadata, or row layout, of all incoming streams must be identical. This includes the number of fields, their names, data types, and their order.

The 'Dummy (do nothing)' step is a common and simple utility step used for this purpose. It can accept multiple incoming hops and acts as a funnel, passing all rows from the input streams sequentially into a single output stream without performing any transformation, thus achieving a union.

A. You can only use the Append streams step to join these two flows.

This is incorrect because other steps, most notably 'Dummy (do nothing)', are also designed to funnel multiple streams into one, effectively performing a union.

D. The row layout can be different between the two steps as long as the data types are the same.

This is incorrect. PDI requires the entire row layout (metadata) to be identical, including field names and order, not just the data types.

1. Hitachi Vantara - Pentaho Documentation

Transformation Step Reference

"Dummy (do nothing)": "This step can be used to fork and join streams. When you direct multiple streams into this step

it acts as a funnel. The streams are not joined

but read one after the other." This confirms the 'Dummy' step's role in uniting flows.

Source: Pentaho Data Integration official documentation

available through the Hitachi Vantara support portal. Specific section: "Transformation Step Reference".

2. Hitachi Vantara - Pentaho Documentation

"Working with Transformations": The documentation on designing transformations consistently emphasizes the concept of stream metadata. It explains that when streams are merged or united

PDI expects the row structure to be consistent. If the metadata differs between incoming hops to a single step (like a Dummy step)

a "We detected rows with varying layout" error will occur.

Source: Pentaho Data Integration official documentation

"Working with Transformations" section

principles of data flow and stream metadata.