Vector rescoring - Simplify code for k == null

[carlosdelest]

Following up on #116663 and #118774

#118774 makes k to be always specified, defaulting to size in case it's null.

This allows to perform some simplifications on rescoring, as we'll always have k to limit the results returned per shard.

[carlosdelest]

Profiling can be simplified, as we always know the number of results to return

[carlosdelest]

We know in advance the number of comparisons done

[carlosdelest]

We always have a specific k, it makes no sense to use parameters.

[carlosdelest]

@elasticmachine update branch

[carlosdelest]

@elasticmachine update branch

[elasticsearchmachine]

Pinging @elastic/es-search-relevance (Team:Search Relevance)

[benwtrent]

Good cleanup. Also, I removed the v8.9 tag, I think you added that by accident.

[benwtrent]

Actually, one of the things I thought we should do is switch to use k as the oversampling and not num candidates. That was one of the original reasons for the overall change of eagerly setting k to request size.

[carlosdelest]

Actually, one of the things I thought we should do is switch to use k as the oversampling and not num candidates. That was one of the original reasons for the overall change of eagerly setting k to request size.

Oh yeah, it all started with this conversation.

I can work on that as a follow up - and get back the original design:

{
    "query": {
        "knn": {
            "field": "emb",
            "query_vector": [...],
            "k": 10,
            "num_candidates": 100,
            "rescore_vector": {
                "oversample": 2.0
            }
        }
    }
}

This will mean rescoring k * oversample from the num_candidates retrieved on each shard, and returning the top k out of them.

I'll merge this and start working on it. Draft PR: #119835

[elasticsearchmachine]

💔 Backport failed

Status	Branch	Result
❌	8.x	Commit could not be cherrypicked due to conflicts

You can use sqren/backport to manually backport by running backport --upstream elastic/elasticsearch --pr 118997

[carlosdelest]

💚 All backports created successfully

Status	Branch	Result
✅	8.x

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