UR2N: Unified Retriever and ReraNker

Abstract

The two-stage retrieval paradigm has gained popularity, where a neural model serves as a re-ranker atop a non-neural first-stage retriever. We argue that this approach, involving two disparate models without interaction, represents a suboptimal choice. To address this, we propose a unified encoder-decoder architecture with a novel training regimen which enables the encoder representation to be used for retrieval and the decoder for re-ranking within a single unified model, facilitating end-to-end retrieval. We incorporate XTR-style retrieval on top of the trained MonoT5 reranker to specifically concentrate on addressing practical constraints to create a lightweight model. Results on the BIER benchmark demonstrate the effectiveness of our unified architecture, featuring a highly optimized index and parameters. It outperforms ColBERT, XTR, and even serves as a superior re-ranker compared to the Mono-T5 reranker. The performance gains of our proposed system in reranking become increasingly evident as model capacity grows, particularly when compared to rerankers operating over traditional first-stage retrievers like BM25. This is encouraging, as it suggests that we can integrate more advanced retrievers to further enhance final reranking performance. In contrast, BM25’s static nature limits its potential for such improvements.