Col-Bandit: Zero-Shot Query-Time Pruning for Late-Interaction Retrieval

TL;DR

Col-Bandit is a zero-shot query-time pruning method that significantly reduces computational costs in late-interaction retrieval systems by adaptively identifying and computing only the necessary token interactions for accurate ranking.

Contribution

It introduces a novel, uncertainty-aware pruning algorithm that sparsifies late-interaction matrices on the fly without requiring offline preprocessing or retraining.

Findings

Reduces MaxSim FLOPs by up to 5× on benchmarks

Preserves ranking quality while pruning interactions

Operates as a zero-shot, drop-in layer for existing systems

Abstract

Multi-vector late-interaction retrievers such as ColBERT achieve state-of-the-art retrieval quality, but their query-time cost is dominated by exhaustively computing token-level MaxSim interactions for every candidate document. While approximating late interaction with single-vector representations reduces cost, it often incurs substantial accuracy loss. We introduce Col-Bandit, a query-time pruning algorithm that reduces this computational burden by casting reranking as a finite-population Top-$K$ identification problem. Col-Bandit maintains uncertainty-aware bounds over partially observed document scores and adaptively reveals only the (document, query token) MaxSim entries needed to determine the top results under statistical decision bounds with a tunable relaxation. Unlike coarse-grained approaches that prune entire documents or tokens offline, Col-Bandit sparsifies the interaction matrix on the fly. It operates as a zero-shot, drop-in layer over standard multi-vector systems, requiring no index modifications, offline preprocessing, or model retraining. Experiments on textual (BEIR) and multimodal (REAL-MM-RAG) benchmarks show that Col-Bandit preserves ranking fidelity while reducing MaxSim FLOPs by up to 5$\times$, indicating that dense late-interaction scoring contains substantial redundancy that can be identified and pruned efficiently at query time.