Re-ranking and Query Transformation

While selecting and optimizing vector stores and indexing strategies are primary considerations for production RAG, achieving high-quality results often requires refining what is searched for and how retrieved results are ranked. Initial retrieval based purely on vector similarity, while efficient, might not always capture the user's full intent or surface the most pertinent information from the candidate set. Techniques to enhance retrieval relevance through query transformation and result re-ranking are analyzed here.

Refining the Search: Query Transformation

User queries can be ambiguous, overly concise, or lack sufficient context for effective semantic search. Query transformation techniques aim to modify the original user query into one or more optimized queries that are more likely to yield relevant documents from the vector store.

Query Expansion

The simplest form involves expanding the query with related terms or concepts, often using an LLM. The goal is to broaden the search aperture slightly to catch documents that might use different terminology for the same idea.

For example, a user query like "RAG performance issues" could be expanded by an LLM to include terms like "Retrieval-Augmented Generation latency", "vector search optimization", "RAG throughput bottlenecks", or "indexing efficiency".

• Implementation: This typically involves a preliminary LLM call before the retrieval step. LangChain's MultiQueryRetriever is a standard implementation that automates this process, generating multiple variations of the query to broaden the search scope.
• Considerations: While potentially helpful, overly aggressive expansion can lead to topic drift, where the expanded query retrieves irrelevant documents. Careful prompt engineering and potentially limiting the scope of expansion are necessary.

Query Decomposition

Complex user questions often contain multiple sub-problems. Trying to answer them with a single retrieval pass can be ineffective. Query decomposition breaks down a complex query into several simpler, independent sub-queries. Each sub-query is executed against the retriever, and the results are then synthesized (often by a final LLM call) to generate the comprehensive answer.

Consider the query: "What are the differences in context window management strategies and persistent memory stores in LangChain?"

This could be decomposed into:

1. "What are context window management strategies in LangChain?"
2. "What are persistent memory stores in LangChain?"
3. "How do context window management and persistent memory differ in LangChain?"

• Implementation: Requires an initial LLM call to perform the decomposition, followed by parallel or sequential retrieval for each sub-query, and a final synthesis step. This logic is often implemented using custom chains or agentic workflows that parse the complex query into a list of standalone questions.
• Considerations: Adds complexity and latency due to multiple retrieval steps and LLM calls. It's most effective for genuinely multifaceted questions.

Document Embeddings (HyDE)

HyDE takes a different approach. Instead of modifying the query text, it uses an LLM to generate a document or answer that perfectly addresses the user's query. This document is then embedded, and its embedding is used to search the vector store. The assumption is that the embedding of a perfect answer will be located closer in the vector space to the embeddings of relevant documents.

• Implementation: Involves an LLM call to generate the document, an embedding step for this document, and then standard vector retrieval using the generated embedding.
• Considerations: Can be surprisingly effective, especially for queries requiring detailed understanding. The quality depends heavily on the LLM's ability to generate a relevant and well-structured response. It adds an LLM call and embedding step before retrieval.

Improving Relevance: Re-ranking Retrieved Results

Initial retrieval methods, like vector similarity search, are optimized for speed and recall over a large corpus. They often return a set of k candidate documents (e.g., top 20). However, the documents most truly relevant to the query might be scattered within this initial set, not necessarily ranked highest. Re-ranking introduces a second, more computationally intensive stage to re-order these initial candidates based on a more fine-grained assessment of relevance.

Modified RAG pipeline incorporating optional query transformation and a mandatory re-ranking stage after initial retrieval.

Cross-Encoder Models

Unlike bi-encoders used in initial retrieval (which embed query and documents independently), cross-encoders process the query and a candidate document together as a single input. This allows the model to directly compare the query and document text, leading to a more accurate relevance score.

• How it works: A pre-trained cross-encoder model takes (query, document_text) as input and outputs a score indicating relevance (e.g., between 0 and 1). You run this scoring process for each of the top-k candidates retrieved initially.
• Models: Examples include models fine-tuned on relevance datasets like MS MARCO (e.g., cross-encoder/ms-marco-MiniLM-L-6-v2) or modern state-of-the-art options like BAAI/bge-reranker. Managed APIs like Cohere Rerank also function as powerful remote cross-encoders, offering high performance without local infrastructure management.
• Implementation: Iterate through the initial k documents, pass each (query, doc) pair to the model, and sort based on the output scores. LangChain's ContextualCompressionRetriever supports both local Hugging Face embeddings and API-based integrations like CohereRerank.
• Considerations: Cross-encoders are significantly slower than bi-encoders because they must process each query-document pair individually. This latency impact means they are typically only applied to a small number of initial candidates (e.g., top 10-50).

LLM-based Re-ranking

You can use a powerful LLM itself to perform the re-ranking. This involves prompting the LLM with the original query and the content of each candidate document (or a relevant snippet) and asking it to assess relevance, perhaps by assigning a score or a categorical judgment (e.g., "highly relevant", "somewhat relevant", "not relevant").

• Implementation: Similar flow to cross-encoders, but instead of calling a specialized model, you make an LLM API call for each candidate document. Careful prompt design is important to elicit consistent and reliable relevance judgments.
• Considerations: Can achieve very high relevance accuracy, potentially capturing subtleties missed by smaller models. However, this is often the most expensive and highest-latency re-ranking option due to multiple LLM calls. Techniques like instructing the LLM to rank a batch of documents simultaneously can mitigate this, but may affect accuracy.

Incorporating Other Signals

Re-ranking isn't limited to semantic relevance. You can fuse the semantic score from a cross-encoder or LLM with other signals:

• Document Freshness: Prioritize more recently updated documents.
• Source Authority: Boost documents from trusted or authoritative sources.
• User Feedback: Incorporate explicit (thumbs up/down) or implicit (click-through rate) feedback if available.
• Diversity: Penalize documents that are too semantically similar to already selected top documents to avoid redundancy.

The final rank can be determined by a weighted combination of these scores. The formula might be simple, like $Score_{final} = w_1 imes Score_{semantic} + w_2 imes Score_{freshness} + ...$, or involve a more complex learning-to-rank (LTR) model trained on your specific data and relevance criteria.

Combining Transformation and Re-ranking

Query transformation and re-ranking are complementary. Transforming the query helps improve the quality of the initial candidate set retrieved by the fast bi-encoder. Re-ranking then carefully sorts this improved candidate set to bring the absolute best matches to the top. Using both can lead to significant improvements in the final context provided to the LLM for generation.

Practical Notes for Production

• Latency Budget: Both query transformation (especially using LLMs) and re-ranking add latency. Profile your pipeline carefully. Applying re-ranking only to the top N (e.g., 10-20) candidates is a common strategy to balance accuracy and speed. Consider smaller, faster cross-encoder models if latency is critical.
• Cost: LLM calls for transformation or re-ranking increase operational costs. Track token usage and explore smaller models or sampling strategies if cost becomes prohibitive.
• Evaluation: Implementing these techniques requires careful evaluation. Measure not just the retrieval metrics (like NDCG@k, Hit Rate@k) after re-ranking, but also the end-to-end quality of the generated answers. Use evaluation sets and potentially A/B testing to confirm that these added complexities genuinely improve the user experience or application performance.

By strategically applying query transformation and re-ranking, you can significantly enhance the relevance and precision of your RAG system, moving from basic semantic similarity to provide more accurate and contextually appropriate information to your LLMs.