[GitHub] [lucene-solr] jpountz commented on a change in pull request #905: LUCENE-8990: Add estimateDocCount(visitor) method to PointValues

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jpountz commented on a change in pull request #905: LUCENE-8990: Add estimateDocCount(visitor) method to PointValues
URL: https://github.com/apache/lucene-solr/pull/905#discussion_r331111490
 
 

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 File path: lucene/core/src/java/org/apache/lucene/index/PointValues.java
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 @@ -241,9 +241,28 @@ default void visit(DocIdSetIterator iterator, byte[] packedValue) throws IOExcep
    * than {@link #intersect(IntersectVisitor)}.
    * @see DocIdSetIterator#cost */
   public long estimateDocCount(IntersectVisitor visitor) {
-    return (long) Math.ceil(estimatePointCount(visitor) / ((double) size() / getDocCount()));
+    long estimatedPointCount = estimatePointCount(visitor);
+    int docCount = getDocCount();
+    double size = size();
+    if (estimatedPointCount >= size) {
+      // math all docs
+      return docCount;
+    } else if (size == docCount || estimatedPointCount == 0L ) {
+      // if the point count estimate is 0 or we have only single values
+      // return this estimate
+      return  estimatedPointCount;
+    } else {
+      // in case of multi values estimate the number of docs using the solution provided in
+      // https://math.stackexchange.com/questions/1175295/urn-problem-probability-of-drawing-balls-of-k-unique-colors
+      // then approximate the solution for points per doc << size() which results in the expression
+      // D * (1 - ((N - n) / N)^(N/D))
 
 Review comment:
   maybe clarify what are D, N and n?

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