DuckDB community extension for prefiltered HNSW using ACORN-1

Name: DuckDB community extension for prefiltered HNSW using ACORN-1
Availability: InStock
Author: cigrainger

by cigrainger·Mar 25, 2026·90 points·5 comments

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AI Analysis

●●●BangerBig BrainDark HorseSolve My Problem

ACORN-1 pushes WHERE filters into HNSW traversal, fixing pgvector's post-filter limitation.

Strengths

•Filter predicates evaluated during HNSW traversal instead of post-filtering results.
•ACORN-1 two-hop expansion recovers graph connectivity under selective filtering.
•Benchmarks show 10/10 results on 1% selectivity filters where upstream returns 0/10.

Weaknesses

•Fork of duckdb-vss requires tracking upstream changes and merging manually.
•ACORN-1 algorithm is recent (2024 paper) with limited production battle-testing.

Post Description

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.

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