LUCENE-9114: Improve ValueSourceScorer's Default Cost Implementation (#1303)

This commit makes ValueSourceScorer's costing algorithm also take the delegated FunctionValues's cost into consideration when calculating its cost. FunctionValues now exposes a cost method which is used by ValueSourceScorer's default matchCost method. In addition, ValueSourceScorer exposes a matchCost method which can be overridden to specify a custom costing mechanism

lucene/CHANGES.txt

@@ -79,6 +79,8 @@ Improvements
   Nepali, Serbian, and Tamil. New stoplist: Indonesian. Adds gradle 'snowball'
   task to regenerate and ease future upgrades. (Robert Muir, Dawid Weiss)
 
+* LUCENE-9114: Improve ValueSourceScorer's Default Cost Implementation (Atri Sharma)
+
 Bug fixes
 
 * LUCENE-8663: NRTCachingDirectory.slowFileExists may open a file while 

lucene/queries/src/java/org/apache/lucene/queries/function/FunctionValues.java

@@ -90,6 +90,16 @@ public abstract class FunctionValues {
    * @return the number of unique sort ordinals this instance has
    */
   public int numOrd() { throw new UnsupportedOperationException(); }
+
+  /**
+   * An estimate of the expected cost to return a value for a document.
+   * It's intended to be used by TwoPhaseIterator.matchCost implementations.
+   * Returns an expected cost in number of simple operations like addition, multiplication,
+   * comparing two numbers and indexing an array.
+   * The returned value must be positive.
+   */
+  public float cost() { return 100; }
+
   public abstract String toString(int doc) throws IOException;
 
   /**
@@ -151,6 +161,10 @@ public abstract class FunctionValues {
       public boolean matches(int doc) {
         return true;
       }
+      @Override
+      public float matchCost() {
+        return 0f;
+      }
     };
   }
 

lucene/queries/src/java/org/apache/lucene/queries/function/ValueSourceScorer.java

@@ -39,6 +39,9 @@ import org.apache.lucene.search.Weight;
  * @lucene.experimental
  */
 public abstract class ValueSourceScorer extends Scorer {
+  // Fixed cost for a single iteration of the TwoPhaseIterator instance
+  private static final int DEF_COST = 5;
+
   protected final FunctionValues values;
   private final TwoPhaseIterator twoPhaseIterator;
   private final DocIdSetIterator disi;
@@ -55,7 +58,7 @@ public abstract class ValueSourceScorer extends Scorer {
 
       @Override
       public float matchCost() {
-        return 100; // TODO: use cost of ValueSourceScorer.this.matches()
+        return ValueSourceScorer.this.matchCost();
       }
     };
     this.disi = TwoPhaseIterator.asDocIdSetIterator(twoPhaseIterator);
@@ -94,4 +97,17 @@ public abstract class ValueSourceScorer extends Scorer {
     return Float.POSITIVE_INFINITY;
   }
 
+  /**
+   * Cost evaluation function which defines the cost of access for the TwoPhaseIterator for this class
+   * This method should be overridden for specifying custom cost methods. Used by {@link TwoPhaseIterator#matchCost()}
+   * for the instance owned by this class
+   *
+   * @return cost of access
+   *
+   * @lucene.experimental
+   */
+  protected float matchCost() {
+    // Cost of iteration is fixed cost + cost exposed by delegated FunctionValues instance
+    return DEF_COST + values.cost();
+  }
 }

lucene/queries/src/test/org/apache/lucene/queries/function/TestFunctionRangeQuery.java

@@ -22,6 +22,8 @@ import org.apache.lucene.index.DirectoryReader;
 import org.apache.lucene.index.IndexReader;
 import org.apache.lucene.index.IndexWriter;
 import org.apache.lucene.index.IndexWriterConfig;
+import org.apache.lucene.search.BooleanClause;
+import org.apache.lucene.search.BooleanQuery;
 import org.apache.lucene.search.Explanation;
 import org.apache.lucene.search.IndexSearcher;
 import org.apache.lucene.search.Query;
@@ -137,6 +139,19 @@ public class TestFunctionRangeQuery extends FunctionTestSetup {
         explain.toString());
   }
 
+  @Test
+  public void testTwoRangeQueries() throws IOException {
+    Query rq1 = new FunctionRangeQuery(INT_VALUESOURCE, 2, 4, true, true);
+    Query rq2 = new FunctionRangeQuery(INT_VALUESOURCE, 8, 10, true, true);
+    Query bq = new BooleanQuery.Builder()
+        .add(rq1, BooleanClause.Occur.SHOULD)
+        .add(rq2, BooleanClause.Occur.SHOULD)
+        .build();
+
+    ScoreDoc[] scoreDocs = indexSearcher.search(bq, N_DOCS).scoreDocs;
+    expectScores(scoreDocs, 10, 9, 8, 4, 3, 2);
+  }
+
   private void expectScores(ScoreDoc[] scoreDocs, int... docScores) {
     assertEquals(docScores.length, scoreDocs.length);
     for (int i = 0; i < docScores.length; i++) {