Hey folks! As someone doing hybrid search daily and wishing I could have a pgvector-like experience but with actual prefiltered approximate nearest neighbours, I decided to just take a punt on implementing ACORN on a fork of the DuckDB VSS extension. I had to make some changes to (vendored) usearch that I'm thinking of submitting upstream. But this does the business. Approximate nearest neighbours with WHERE prefiltering.
Edit: Just to clarify, this has been accepted into the community extensions repo. So you can use it like:
```
INSTALL hnsw_acorn FROM community;
LOAD hnsw_acorn;
```
Curious about filter selectivity though: does ACORN-1 still help when the filter eliminates, say, 95% of the index? I've seen cases where tight category filters completely undermine graph traversal because the graph structure assumed a uniform distribution over the subspace.
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