Revert "fix: stop underestimating is_empty / not-null cardinality by 1/3 (#10128)" (#10140)

This reverts commit 50fe2e8139.

lib/segment/src/index/field_index/null_index/mutable_null_index/mod.rs

@@ -76,10 +76,6 @@ mod tests {
 
         let filter_is_null = FieldCondition::new_is_null(key.clone(), true);
 
-        let filter_is_not_null = FieldCondition::new_is_null(key.clone(), false);
-
-        let filter_is_empty = FieldCondition::new_is_empty(key.clone(), true);
-
         let filter_is_not_empty = FieldCondition {
             key: key.clone(),
             r#match: None,
@@ -156,22 +152,9 @@ mod tests {
             .estimate_cardinality(&filter_is_not_empty, &hw_cell)
             .unwrap()
             .unwrap();
-        let is_empty_cardinality = null_index
-            .estimate_cardinality(&filter_is_empty, &hw_cell)
-            .unwrap()
-            .unwrap();
-        let not_null_cardinality = null_index
-            .estimate_cardinality(&filter_is_not_null, &hw_cell)
-            .unwrap()
-            .unwrap();
 
         assert_eq!(is_null_cardinality.exp, 50);
         assert_eq!(non_empty_cardinality.exp, 50);
-        // Complement estimates use total − indexed count (no arbitrary delete discount).
-        assert_eq!(is_empty_cardinality.exp, 50);
-        assert_eq!(is_empty_cardinality.max, 50);
-        assert_eq!(not_null_cardinality.exp, 50);
-        assert_eq!(not_null_cardinality.max, 50);
     }
 
     #[test]

lib/segment/src/index/field_index/null_index/read_ops.rs

@@ -180,12 +180,11 @@ pub(super) fn estimate_cardinality<N: NullIndexRead>(
     let estimation = if let Some(is_empty) = is_empty {
         if *is_empty {
             let has_values_count = has_values_flags.count_trues()?;
-            // Upper bound: may include soft-deleted offsets (index has no deleted bitslice).
             let estimated = total_point_count.saturating_sub(has_values_count);
 
             CardinalityEstimation {
                 min: 0,
-                exp: estimated,
+                exp: 2 * estimated / 3, // assuming 1/3 of the points are deleted
                 max: estimated,
                 primary_clauses: vec![PrimaryCondition::from(FieldCondition::new_is_empty(
                     key.clone(),
@@ -206,12 +205,11 @@ pub(super) fn estimate_cardinality<N: NullIndexRead>(
             ))
         } else {
             let is_null_count = is_null_flags.count_trues()?;
-            // Upper bound: may include soft-deleted offsets (index has no deleted bitslice).
             let estimated = total_point_count.saturating_sub(is_null_count);
 
             CardinalityEstimation {
                 min: 0,
-                exp: estimated,
+                exp: 2 * estimated / 3, // assuming 1/3 of the points are deleted
                 max: estimated,
                 primary_clauses: vec![PrimaryCondition::from(FieldCondition::new_is_null(
                     key.clone(),