[LUCENE-8177] BlockMaxConjunctionScorer should compute better lower bounds of the required scores

• Type: Bug
• Status: Open
• Priority: Major
• Resolution: Unresolved
• Lucene Fields: New
• Assignee: Unassigned
• Reporter: Adrien Grand
• Created: 19/Feb/18 10:41
• Updated: 28/Aug/22 15:25

Description

Assuming N scorers, BlockMaxConjunctionScorer computes a lower bound of the sum of scores for scorers 0..i, for any given 0 <= i < N.

For instance say you are searching for "quick AND fox", that a hit needs a score of 4 to be competitive and that "quick" contributes at most 3 to the score and "fox" 2. This means that for a given hit to be competitive, the sum of scores must be at least 4-maxScore(fox)=4-2=2 after having scored "quick" and 4 after having scored "fox".

Currently we have this in BlockMaxConjunctionScorer:

// Also compute the minimum required scores for a hit to be competitive
// A double that is less than 'score' might still be converted to 'score'
// when casted to a float, so we go to the previous float to avoid this issue
minScores[minScores.length - 1] = minScore > 0 ? Math.nextDown(minScore)

We currently use Math.minDown(float) to be safe, but we would get a better bound by computing the lowest double that is converted to minScore when casted to a float.