What is query expansion and how does it improve video search recall?

Query Expansion and Its Role in Video Search Recall

Query expansion is a technique used in search systems to enhance a user’s original query by adding related terms, synonyms, or contextual variations. The goal is to address the limitations of short or ambiguous search terms, which often fail to capture the full scope of relevant content. For example, a user searching for “car racing” might benefit from an expanded query that includes terms like “Formula 1,” “NASCAR,” or “motorsport.” In video search, this is particularly useful because video metadata (titles, descriptions, tags) can be inconsistent or sparse. By broadening the query, the system increases the likelihood of matching videos that use alternative terminology to describe the same concept.

Improving Recall Through Broader Matching Recall in search refers to the system’s ability to retrieve all relevant content. Without query expansion, a search for “bike tricks” might miss videos tagged with “BMX stunts” or “cycling freestyle” because the exact keywords don’t match. By automatically adding synonyms or related phrases, the search covers a wider range of metadata, capturing videos that would otherwise be overlooked. For instance, expanding “AI tutorial” to include “machine learning guide” or “deep learning lecture” ensures videos using those terms are included. This is critical in video platforms where creators might use informal or niche terminology (e.g., “unboxing” vs. “product review”). Query expansion bridges this vocabulary gap, directly improving recall by casting a wider semantic net.

Implementation Methods and Trade-offs Developers can implement query expansion using methods like synonym lists (e.g., WordNet), embedding-based models (Word2Vec, BERT), or pseudo-relevance feedback (analyzing top results to extract additional terms). For example, a system might use a pre-trained language model to generate contextually similar terms for “football” (e.g., “soccer,” “Premier League”). However, there’s a trade-off: over-expanding queries can reduce precision by including irrelevant results. To mitigate this, techniques like weighted term boosting (prioritizing original keywords) or filtering based on co-occurrence statistics are used. Tools like Elasticsearch offer built-in query expansion features, allowing developers to experiment with parameters like synonym weighting. While the primary benefit is improved recall, careful tuning ensures the balance between coverage and relevance remains practical for users.