I have been summoned. Hey it's David from the podcast. As someone who builds search for users every day and shaped the user experience for vector search at OpenSearch I assure you no one is afraid of their technology becoming inferior.

There are two components of search that are really important to understand why BM25 (will likely) not go away for a long time. The first is precision and the second is recall. Precision is the measure of how many relevant results were returned in light of all the results returned. A completely precise search would return only the relevant results and no irrelevant results.

Recall on the other hand measures how many of all the relevant results were returned. For example, if our search only returns 5 results but we know that there were 10 relevant search results that should have been returned we would say the recall is 50%.

These two components are always at odds with each other. Vector search excels at increasing recall. It is able to find documents that are semantically similar. The problem with this is semantically similar documents might not actually be what the user is looking for. This is because vectors are only a representation of user intent.

Heres an example: A user looks up "AWS Config". Vector search would read this and may rate it as similar to ["amazon web services configuration", "cloud configuration", "infrastructure as a service setup"]. In this case the user was looking for a file called, "AWS.config". Vector search is inherently imprecise. It is getting better but it's not replacing BM25 as a scoring mechanism any time soon.

You don't have to believe me though. Weaviate, Vespa, Qdrant all support BM25 search for a reason. Here is an in depth blog that dives more into hybrid search: https://opensearch.org/blog/hybrid-search/

As an aside, vector search is also much more expensive than BM25. It's very hard to scale and get precise results.