perf: tighten WAND block score upper bound

rust/lance-index/src/scalar/inverted/wand.rs

@@ -148,7 +148,7 @@ impl PostingIterator {
         num_doc: usize,
     ) -> Self {
         let approximate_upper_bound = match list.max_score() {
-            Some(max_score) => max_score, // the index doesn't include the full BM25 upper bound at indexing time, so we need to multiply it here
+            Some(max_score) => max_score,
             None => idf(list.len(), num_doc) * (K1 + 1.0),
         };
 
@@ -275,7 +275,7 @@ impl PostingIterator {
     #[inline]
     fn block_max_score(&self) -> f32 {
         match self.list {
-            PostingList::Compressed(ref list) => list.block_max_score(self.block_idx) * (K1 + 1.0),
+            PostingList::Compressed(ref list) => list.block_max_score(self.block_idx),
             PostingList::Plain(_) => self.approximate_upper_bound,
         }
     }
@@ -993,4 +993,23 @@ mod tests {
 
         assert!(result.is_ok());
     }
+
+    #[test]
+    fn test_block_max_score_matches_stored_value() {
+        let doc_ids = vec![0_u32];
+        let block_max_scores = vec![0.7_f32];
+        let posting_list = generate_posting_list(doc_ids, 0.7, Some(block_max_scores), true);
+        let expected = match &posting_list {
+            PostingList::Compressed(list) => list.block_max_score(0),
+            PostingList::Plain(_) => unreachable!("expected compressed posting list"),
+        };
+
+        let posting = PostingIterator::new(String::from("test"), 0, 0, posting_list, 1);
+
+        let actual = posting.block_max_score();
+        assert!(
+            (actual - expected).abs() < 1e-6,
+            "block max score should match stored value"
+        );
+    }
 }