Control the auto-adjustment of targetHits in ANN using post-filtering.

When searching the HNSW index in a post-filtering case,
targetHits is auto-adjusted in an effort to still expose targetHits hits to first-phase ranking after post-filtering.
The following formula is now used to ensure an upper bound of adjustedTargetHits,
avoiding that the search in the HNSW index takes too long.

adjustedTargetHits = min(targetHits / estimatedHitRatio, targetHits * targetHitsMaxAdjustmentFactor).

The target-hits-max-adjustment-factor can be set in a rank profile and overriden per query.
The value is in the range [1.0,inf], with the default being 20.0.

When setting this to 1.0, auto-adjustment of targetHits is effectively disabled.

1 files changed, 9 insertions, 0 deletions

diff --git a/config-model/src/main/java/com/yahoo/schema/RankProfile.java b/config-model/src/main/java/com/yahoo/schema/RankProfile.java
index 69f32daef4a..35ef12f077a 100644
--- a/config-model/src/main/java/com/yahoo/schema/RankProfile.java
+++ b/config-model/src/main/java/com/yahoo/schema/RankProfile.java
@@ -100,6 +100,7 @@ public class RankProfile implements Cloneable {
     private Double termwiseLimit = null;
     private Double postFilterThreshold = null;
     private Double approximateThreshold = null;
+    private Double targetHitsMaxAdjustmentFactor = null;
 
     /** The drop limit used to drop hits with rank score less than or equal to this value */
     private double rankScoreDropLimit = -Double.MAX_VALUE;
@@ -768,6 +769,7 @@ public class RankProfile implements Cloneable {
     public void setTermwiseLimit(double termwiseLimit) { this.termwiseLimit = termwiseLimit; }
     public void setPostFilterThreshold(double threshold) { this.postFilterThreshold = threshold; }
     public void setApproximateThreshold(double threshold) { this.approximateThreshold = threshold; }
+    public void setTargetHitsMaxAdjustmentFactor(double factor) { this.targetHitsMaxAdjustmentFactor = factor; }
 
     public OptionalDouble getTermwiseLimit() {
         if (termwiseLimit != null) return OptionalDouble.of(termwiseLimit);
@@ -789,6 +791,13 @@ public class RankProfile implements Cloneable {
         return uniquelyInherited(p -> p.getApproximateThreshold(), l -> l.isPresent(), "approximate-threshold").orElse(OptionalDouble.empty());
     }
 
+    public OptionalDouble getTargetHitsMaxAdjustmentFactor() {
+        if (targetHitsMaxAdjustmentFactor != null) {
+            return OptionalDouble.of(targetHitsMaxAdjustmentFactor);
+        }
+        return uniquelyInherited(p -> p.getTargetHitsMaxAdjustmentFactor(), l -> l.isPresent(), "target-hits-max-adjustment-factor").orElse(OptionalDouble.empty());
+    }
+
     /** Whether we should ignore the default rank features. Set to null to use inherited */
     public void setIgnoreDefaultRankFeatures(Boolean ignoreDefaultRankFeatures) {
         this.ignoreDefaultRankFeatures = ignoreDefaultRankFeatures;