Both Sub-Question Query Engine and multi-step query decomposition in RAG tackle complex queries, but they approach the problem from different angles:

Sub-Question Query Engine:

It focuses on divide and conquer approach. It Decomposes a complex query into a series of smaller, focused sub-questions.

Each sub-question is sent to a dedicated Sub-Question Query Engine that retrieves relevant information from its specific data source.

Hence, it ensures each sub-question gets the appropriate data source, leading to more precise results.

Multi-step query decomposition:

It focuses on sequential refinement of the query. It breaks down the complex query into intermediate steps, progressively enriching the search with the retrieved information.

Each step searches for relevant documents based on the current query state, updating the query with extracted knowledge.

Hence, it avoids redundant retrieval by refining the query with each step.

Hope it answers your question :)