Show HN: PageIndex – Vectorless RAG (github.com)

What about latency?

So if I understand this correctly it goes over every possible document with an LLM each time someone performs a search?

I might have misunderstood of course.

If so, then the use cases for this would be fairly limited since you'd have to deal with lots of latency and costs. In some cases (legal documents, medical records, etc) it might be worth it though.

An interesting alternative I've been meaning to try out is inverting this flow. Instead of using an LLM at time of searching to find relevant pieces to the query, you flip it around: at time of ingesting you let an LLM note all of the possible questions that you can answer with a given text and store those in an index. You could them use some traditional full-text search or other algorithms (BM25?) to search for relevant documents and pieces of text. You could even go for a hybrid approach with vectors on top or next to this. Maybe vectors first and then more ranking with something more traditional.

What appeals to me with that setup is low latency and good debug-ability of the results.

But as I said, maybe I've misunderstood the linked approach.

Very cool. These days I’m building RAG over a large website, and when I look at the results being fed into the LLM, most of them are so silly it’s surprising the LLM even manages to extract something meaningful. Always makes me wonder if it’s just using prior knowledge even though it’s instructed not to do so (which is hacky).

I like your approach because it seems like a very natural search process, like a human would navigate a website to find information. I imagine the tradeoff is performance of both indexing and search, but for some use cases (like mine) it’s a good sacrifice to make.

I wonder if it’s useful to merge to two approaches. Like you could vectorize the nodes in the tree to give you a heuristic that guides the search. Could be useful in cases where information is hidden deep in a subtree, in a way that the document’s structure doesn’t give it away.

This is good for applications where a background queue based RAG is acceptable. You upload a file, set the expectation to the user that you're processing it and needs more time for a few hours and then after X hours you deliver them. Great for manuals, documentation and larger content.

But for on-demand, near instant RAG (like say in a chat application), this won't work. Speed vs accuracy vs cost. Cost will be a really big one.

[dead]

This will scale when you have a single/a small set of document(s) and want your questions answered.

When you have a question and you don't know which of the million documents in your dataspace contains the answer - I'm not sure how this approach will perform. In that case we are looking at either feeding an enormously large tree as context to LLM or looping through potentially thousands of iterations between a tree & a LLM.

That said, this really is a good idea for a small search space (like a single document).

Not sure if I fully understand it, but this seems highly inefficient?

Instead of using embeddings which are easy to make a cheap to compare, you use summarized sections of documents and process them with an LLM? LLM's are slower and more expensive to run.

Unrelated: why is chat search in Claude so bad?

[dead]

I let a boot do a free text search over and indexed database. Works ok. I've also tried keyword based retrieval and vector search.

I've found all leave something to be desired, sadly.

There's good reasons to do this. Embedding similarity is _not_ a reliable method of determining relevance.

I did some measurements and found you can't even really tell if two documents are "similar" or not. Here: https://joecooper.me/blog/redundancy/

One common way is to mix approaches. e.g. take a large top-K from ANN on embeddings as a preliminary shortlist, then run a tuned LLM or cross encoder to evaluate relevance.

I'll link here these guys' paper which you might find fun: https://arxiv.org/pdf/2310.08319

At the end of the day you just want a way to shortlist and focus information that's cheaper, computationally, and more reliable, than dumping your entire corpus into a very large context window.

So what we're doing is fitting the technique to the situation. Price of RAM; GPU price; size of dataset; etc. The "ideal" setup will evolve as the cost structure and model quality evolves, and will always depend on your activity.

But for sure, ANN-on-embedding as your RAG pipeline is a very blunt instrument and if you can afford to do better you can usually think of a way.

vectorless rag? I think I have one of those in my kitchen

Sounds a bit like generative retrieval (e.g. this Google paper here: https://arxiv.org/abs/2202.06991)

Looks like this should scale spectacularly poorly.

Might be useful for a few hundred documents max though.

"Human-like Retrieval: Simulates how human experts navigate and extract knowledge from complex documents." - pretty sure I use control-f when I look for stuff

I have a RAG built on 10000+ docs knowledge base. On vector store, of course (Qdrant - hybrid search). It work smoothly and quite reliable.

I wonder how this "vectorless" engine would deal with this. Simply, I can't see this tech scalable.

This seems really interesting but I can't quite figure out if this is like a SaaS product or an OSS library? The code sample seems to indicate that it uses some sort of "client" to send the document somewhere and then wait to retrieve it later.

But the home page doesn't indicate any sort of sign up or pricing.

So I'm a little confused.

edit Ok I found a sign up flow, but the verification email never came :(

Context and prompt engineering is the most important of AI, hands down.

There are plenty of lightweight retrieval options that don't require a separate vector database (I'm the author of txtai [https://github.com/neuml/txtai], which is one of them).

It can be as simple this in Python: you pass an index operation a data generator and save the index to a local folder. Then use that for RAG.

My approach in "LLM-only RAG for small corpora" [0] was to mechanically make an outline version of all the documents _without_ an LLM, feed that to an LLM with the prompt to tell which docs are likely relevant, and then feed the entirety of those relevant docs to a second LLM call to answer the prompt. It only works with markdown and asciidoc files, but it's surprisingly solid for, for example, searching a local copy of the jj or helix docs. And if the corpus is small enough and your model is on the cheap side (like Gemini 2.5 Flash), you can of course skip the retrieval step and just send the entire thing every time.

[0]: https://crespo.business/posts/llm-only-rag/

This is like semantic version of B+ trees.

> It moves RAG away from approximate "semantic vibes" and toward explicit reasoning about where information lives. That clarity can help teams trust outputs and debug workflows more effectively.

Wasn't this a feature of RAGs, though? That they could match semantics instead of structure, while us mere balls of flesh need to rely on indexes. I'd be interested in benchmarks of this versus traditional vector-based RAGs, is something to that effect planned?

>"Retrieval based on reasoning — say goodbye to approximate semantic search ("vibe retrieval"

How is this not precisely "vibe retrieval" and much more approximate, where approximate in this case is uncertainty over the precise reasoning?

Similarity with conversion to high-dimensional vectors and then something like kNN seems significantly less approximate, less "vibe" based, than this.

This also appears to be completely predicated on pre-enrichment of the documents by adding structure through API calls to, in the example, openAI.

It doesn't at all seem accurate to:

1: Toss out mathematical similarity calculations

2: Add structure with LLMs

3: Use LLMs to traverse the structure

4: Label this as less vibe-ish

Also for any sufficiently large set of documents, or granularity on smaller sets of documents, scaling will become problematic as the doc structure approaches the context limit of the LLM doing the retrieval.

an effective "vectorless RAG" is to have an LLM write search queries against the documents. e.g. if you store your documents in postgres, allow the LLM to construct a regex string that will find relevant matches. If you were searching for “Martin Luther King Jr.”, it might write something like:

    SELECT id, body
    FROM docs
    WHERE body ~* E'(?x)                                     -- x = allow whitespace/comments
      (?:\\m(?:dr|rev(?:erend)?)\\.?\\M[\\s.]+)?             -- optional title: Dr., Rev., Reverend
      (                                                      -- name forms
        (?:\\mmartin\\M[\\s.]+(?:\\mluther\\M[\\s.]+)?\\mking\\M)  -- "Martin (Luther)? King"
      | (?:\\mm\\.?\\M[\\s.]+(?:\\ml\\.?\\M[\\s.]+)?\\mking\\M)     -- "M. (L.)? King" / "M L King"
      | (?:\\mmlk\\M)                                       -- "MLK"
      )
      (?:[\\s.,-]*\\m(?:jr|junior)\\M\\.?)*                  -- optional suffix(es): Jr, Jr., Junior
    ';

I don't see this scaling: https://deepwiki.com/search/how-is-the-tree-formed-and-tra_9...

I'd do some large scale benchmarks before doubling down on this approach.