ES|QL - Improve prefiltering in KNN function

[carlosdelest]

KNN function performs prefiltering, so for a query like:

FROM test
WHERE KNN(field, [0, 1, 2]) AND text: "value"

the KNN function will receive text: "value" as a prefilter:

FROM test
WHERE KNN(field, [0, 1, 2] <with text:"value" as prefilter>) AND text: "value"

However, text: "value" remains as part of the conjunction, so in the end it will be transformed into a boolean query for the KNN and MATCH function. And the problem is, that the MATCH function will be scored, so it will impact both performance and overall scoring.

Including a prefilter for KNN is equivalent to the conjunction itself, so we can safely remove the other side of the conjunction when there is a KNN function on the other side that includes it as prefilter.

This PR transforms the above expression into:

FROM test
WHERE KNN(field, [0, 1, 2] <with text:"value" as prefilter>)

so there is no need to generate a separate query for the MATCH function, and it is not combined for scoring purposes.

[elasticsearchmachine]

Hi @carlosdelest, I've created a changelog YAML for you.