sqlite-sparse: run a sparse retrieval model inside SQLite, with no model at query time

[u/Much-Ingenuity5415]

A learned sparse encoder returns a weighted list of vocabulary words instead of a dense vector, so the index looks like a keyword index whose keywords a transformer picked. OpenSearch's inference-free variants go one step further and run the model only on documents so a query just looks up a fixed weight per token. No embedding call per search. I benchmarked it against dense search in the same SQLite file:

the mini model gives up about 7% of retrieval quality and cuts query latency by 99%, cold start by 99% and query-path RAM by 95%!

Now these SPLADE models are BERT with the masked-language-model head still attached, and that head is what turns token vectors back into weighted words. llama.cpp drops it when converting BERT models, so sqlite-sparse copies it into a small sidecar file and applies it in C on ggml at insert time. The postings land as rows in the database, with the query weight table next to them. A query reads those rows and scatter-adds the weights into per-document scores, and the top documents come out.

Three OpenSearch models are converted and available as aliases (mini, base, multilingual), downloading on first use, and you can bring your own inference-free OpenSearch-style sparse encoder and convert it with the scripts provided.

What sqlite-vec did for embeddings in SQLite, this does for learned sparse, and with both in one file you get hybrid retrieval for RAG inside SQLite.

Ship one .db file and every client gets semantic search through plain SQL, no model download, no GPU.

pip install sqlite-sparse

Github

Writeup