Crossencoder re-ranking

Short Course Q&A Advanced Retrieval for AI with Chroma
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Hi all,

I have 2 questions about crossencoder re-ranking:

The first is about the principle itself. I have not fully understood why it works, because isn’t the purpose of the vector databases to find the most relevant texts (by a distance metrics like cosine-similarity in embedding space)? Is it because we use a different model for reranking and thus get a “second opinion” of the relevance? Or is it because cross-encoders can do something fundamentally different than embedding models?

Second question is about multi-language support. I successfully query English texts by using the embeddings of a German or French query, using text-embedding-ada-002. Seems to work because in an oversimplified view, the language is just 1 dimension out of the 1535 dimensions of text-embedding-ada-002.
Now what about the used cross-encoder, “ms-marco-MiniLM-L-6-v2”, is it able to properly rank if the query and the texts are in different languages? If not, what multilanguage-crossencoders are out there?

Regards,

Thomas

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Hi, I’m also curious about the first question, and would like know why not use cross-encoders to find the most relevant texts in the first place?

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Hi,

This I can answer: Performance.

The crossencoder is slow, running each pair of question/possible answer through the crossencoder takes forever if you have thousands or even millions of documents.
In contrast in the case of precomputed embeddings it is very fast as all it needs is a vector product to calculate the cosine similarity and there are various optimized algorithms around.

Thomas