What are the advantages and disadvantages of retrieving a large number of documents (say top-10 or top-20) versus only a few top relevant ones (top-3) as context for the LLM?

Retrieving a large number of documents (e.g., top-10 or top-20) versus a smaller set (e.g., top-3) as context for an LLM involves trade-offs between breadth of information, computational efficiency, and relevance. The optimal choice depends on the specific use case, the quality of retrieved documents, and the LLM’s capacity to process information effectively.

Advantages of Retrieving More Documents A larger document set provides broader context, which can help the LLM generate more comprehensive answers. For example, in a question about climate change impacts, retrieving 20 documents might include data on temperature trends, regional effects, and mitigation strategies, allowing the model to synthesize diverse perspectives. This reduces the risk of missing critical information that a smaller set might exclude. Additionally, if the retrieval system isn’t perfectly accurate, including more documents can compensate for minor errors in ranking—for instance, a lower-ranked document might contain a key detail the top-3 lack. However, this assumes the additional documents are at least somewhat relevant; irrelevant content could introduce noise.

Disadvantages of Retrieving More Documents Processing more documents increases computational costs and latency. LLMs have token limits, so including 20 documents may force truncation, discarding parts of the context. For example, if each document is 500 tokens, 20 documents consume 10,000 tokens, leaving little room for the actual query or response. Irrelevant documents also risk confusing the model. Suppose a user asks about Python debugging, and the top-10 include three outdated Stack Overflow threads; the LLM might prioritize obsolete solutions. Additionally, longer contexts can lead to “lost in the middle” behavior, where the model struggles to focus on the most critical information amid excessive text.

When to Use Fewer Documents Retrieving only top-3 documents works best when precision is prioritized over breadth. For straightforward queries like “How to install library X,” the top-3 results are likely sufficient and minimize noise. This approach is computationally efficient, reducing token usage and costs. It’s also less prone to conflicting information—for example, if the top-3 agree on a method, the LLM can confidently generate a clear answer. However, this strategy assumes the retrieval system is highly accurate. If the top-3 are biased or incomplete (e.g., missing a critical security patch note), the LLM’s output will reflect those gaps. Testing is key: developers should evaluate whether their retrieval system consistently surfaces high-quality results in the top few ranks before limiting context.

In summary, larger document sets offer breadth but risk inefficiency and noise, while smaller sets prioritize precision but require high retrieval accuracy. The decision should align with the application’s goals, resource constraints, and the reliability of the underlying retrieval system.