diff --git a/src/main/java/org/apache/datasketches/tuple/QuickSelectSketch.java b/src/main/java/org/apache/datasketches/tuple/QuickSelectSketch.java
index e32b73577..a50d92046 100644
--- a/src/main/java/org/apache/datasketches/tuple/QuickSelectSketch.java
+++ b/src/main/java/org/apache/datasketches/tuple/QuickSelectSketch.java
@@ -172,11 +172,6 @@ private enum Flags { IS_BIG_ENDIAN, IS_IN_SAMPLING_MODE, IS_EMPTY, HAS_ENTRIES,
       count = mem.getInt(offset);
       offset += Integer.BYTES;
     }
-    //    if (version == serialVersionWithSummaryFactoryUID) {
-    //      final DeserializeResult<SummaryFactory<S>> factoryResult =
-    //          SerializerDeserializer.deserializeFromMemory(mem, offset);
-    //      offset += factoryResult.getSize();
-    //    }
     final int currentCapacity = 1 << lgCurrentCapacity_;
     keys_ = new long[currentCapacity];
     for (int i = 0; i < count; i++) {
diff --git a/src/main/java/org/apache/datasketches/tuple/Union.java b/src/main/java/org/apache/datasketches/tuple/Union.java
index 6b024c1dd..7800b55fa 100644
--- a/src/main/java/org/apache/datasketches/tuple/Union.java
+++ b/src/main/java/org/apache/datasketches/tuple/Union.java
@@ -19,8 +19,13 @@
 
 package org.apache.datasketches.tuple;
 
+import static java.lang.Math.min;
 import static org.apache.datasketches.Util.DEFAULT_NOMINAL_ENTRIES;
 
+import java.lang.reflect.Array;
+
+import org.apache.datasketches.QuickSelect;
+
 /**
  * Compute a union of two or more tuple sketches.
  * A new instance represents an empty set.
@@ -29,10 +34,10 @@
  * @param <S> Type of Summary
  */
 public class Union<S extends Summary> {
-  private final int nomEntries_;
   private final SummarySetOperations<S> summarySetOps_;
   private QuickSelectSketch<S> sketch_;
   private long theta_; // need to maintain outside of the sketch
+  private boolean isEmpty_;
 
   /**
    * Creates new instance with default nominal entries
@@ -49,10 +54,10 @@ public Union(final SummarySetOperations<S> summarySetOps) {
    * @param summarySetOps instance of SummarySetOperations
    */
   public Union(final int nomEntries, final SummarySetOperations<S> summarySetOps) {
-    nomEntries_ = nomEntries;
     summarySetOps_ = summarySetOps;
-    sketch_ = new QuickSelectSketch<S>(nomEntries, null);
+    sketch_ = new QuickSelectSketch<>(nomEntries, null);
     theta_ = sketch_.getThetaLong();
+    isEmpty_ = true;
   }
 
   /**
@@ -60,31 +65,72 @@ public Union(final int nomEntries, final SummarySetOperations<S> summarySetOps)
    * @param sketchIn input sketch to add to the internal set
    */
   public void update(final Sketch<S> sketchIn) {
-    if (sketchIn == null || sketchIn.isEmpty()) { return; }
+    if ((sketchIn == null) || sketchIn.isEmpty()) { return; }
+    isEmpty_ = false;
     if (sketchIn.theta_ < theta_) { theta_ = sketchIn.theta_; }
     final SketchIterator<S> it = sketchIn.iterator();
     while (it.next()) {
       sketch_.merge(it.getKey(), it.getSummary(), summarySetOps_);
     }
+    if (sketch_.theta_ < theta_) {
+      theta_ = sketch_.theta_;
+    }
   }
 
   /**
    * Gets the internal set as a CompactSketch
    * @return result of the unions so far
    */
+  @SuppressWarnings("unchecked")
   public CompactSketch<S> getResult() {
-    sketch_.trim();
-    if (theta_ < sketch_.theta_) {
-      sketch_.setThetaLong(theta_);
-      sketch_.rebuild();
+    if (isEmpty_) {
+      return sketch_.compact();
+    }
+    if ((theta_ >= sketch_.theta_) && (sketch_.getRetainedEntries() <= sketch_.getNominalEntries())) {
+      return sketch_.compact();
+    }
+    long theta = min(theta_, sketch_.theta_);
+
+    int num = 0;
+    {
+      final SketchIterator<S> it = sketch_.iterator();
+      while (it.next()) {
+        if (it.getKey() < theta) { num++; }
+      }
     }
-    return sketch_.compact();
+    if (num == 0) {
+      return new CompactSketch<>(null, null, theta, isEmpty_);
+    }
+    if (num > sketch_.getNominalEntries()) {
+      final long[] keys = new long[num]; // temporary since the order will be destroyed by quick select
+      final SketchIterator<S> it = sketch_.iterator();
+      int i = 0;
+      while (it.next()) {
+        if (it.getKey() < theta) { keys[i++] = it.getKey(); }
+      }
+      theta = QuickSelect.select(keys, 0, num - 1, sketch_.getNominalEntries());
+      num = sketch_.getNominalEntries();
+    }
+    final long[] keys = new long[num];
+    final S[] summaries = (S[]) Array.newInstance(sketch_.summaries_.getClass().getComponentType(), num);
+    final SketchIterator<S> it = sketch_.iterator();
+    int i = 0;
+    while (it.next()) {
+      if (it.getKey() < theta) {
+        keys[i] = it.getKey();
+        summaries[i] = (S) it.getSummary().copy();
+        i++;
+      }
+    }
+    return new CompactSketch<>(keys, summaries, theta, isEmpty_);
   }
 
   /**
    * Resets the internal set to the initial state, which represents an empty set
    */
   public void reset() {
-    sketch_ = new QuickSelectSketch<S>(nomEntries_, null);
+    sketch_.reset();
+    theta_ = sketch_.getThetaLong();
+    isEmpty_ = true;
   }
 }
diff --git a/src/main/java/org/apache/datasketches/tuple/UpdatableSketch.java b/src/main/java/org/apache/datasketches/tuple/UpdatableSketch.java
index dd07d428e..174e28232 100644
--- a/src/main/java/org/apache/datasketches/tuple/UpdatableSketch.java
+++ b/src/main/java/org/apache/datasketches/tuple/UpdatableSketch.java
@@ -55,8 +55,8 @@ public class UpdatableSketch<U, S extends UpdatableSummary<U>> extends QuickSele
    * <a href="{@docRoot}/resources/dictionary.html#p">See Sampling Probability</a>
    * @param summaryFactory An instance of a SummaryFactory.
    */
-  public UpdatableSketch(final int nomEntries, final int lgResizeFactor, final float samplingProbability,
-      final SummaryFactory<S> summaryFactory) {
+  public UpdatableSketch(final int nomEntries, final int lgResizeFactor,
+      final float samplingProbability, final SummaryFactory<S> summaryFactory) {
     super(nomEntries, lgResizeFactor, samplingProbability, summaryFactory);
   }
 
diff --git a/src/main/java/org/apache/datasketches/tuple/adouble/DoubleSketch.java b/src/main/java/org/apache/datasketches/tuple/adouble/DoubleSketch.java
index 57cc8e6d8..92e2cbe98 100644
--- a/src/main/java/org/apache/datasketches/tuple/adouble/DoubleSketch.java
+++ b/src/main/java/org/apache/datasketches/tuple/adouble/DoubleSketch.java
@@ -35,7 +35,26 @@ public class DoubleSketch extends UpdatableSketch<Double, DoubleSummary> {
    * @param mode The DoubleSummary mode to be used
    */
   public DoubleSketch(final int lgK, final DoubleSummary.Mode mode) {
-    super(1 << lgK, ResizeFactor.X8.ordinal(), 1.0F, new DoubleSummaryFactory(mode));
+    this(lgK, ResizeFactor.X8.ordinal(), 1.0F, mode);
+  }
+
+  /**
+   * Creates this sketch with the following parameters:
+   * @param lgK Log_base2 of <i>Nominal Entries</i>.
+   * @param lgResizeFactor log2(resizeFactor) - value from 0 to 3:
+   * <pre>
+   * 0 - no resizing (max size allocated),
+   * 1 - double internal hash table each time it reaches a threshold
+   * 2 - grow four times
+   * 3 - grow eight times (default)
+   * </pre>
+   * @param samplingProbability
+   * <a href="{@docRoot}/resources/dictionary.html#p">See Sampling Probability</a>
+   * @param mode The DoubleSummary mode to be used
+   */
+  public DoubleSketch(final int lgK, final int lgResizeFactor, final float samplingProbability,
+      final DoubleSummary.Mode mode) {
+    super(1 << lgK, lgResizeFactor, samplingProbability, new DoubleSummaryFactory(mode));
   }
 
   /**
diff --git a/src/main/java/org/apache/datasketches/tuple/aninteger/IntegerSketch.java b/src/main/java/org/apache/datasketches/tuple/aninteger/IntegerSketch.java
index 9d759125b..03ca7d093 100644
--- a/src/main/java/org/apache/datasketches/tuple/aninteger/IntegerSketch.java
+++ b/src/main/java/org/apache/datasketches/tuple/aninteger/IntegerSketch.java
@@ -35,7 +35,26 @@ public class IntegerSketch extends UpdatableSketch<Integer, IntegerSummary> {
    * @param mode The IntegerSummary mode to be used
    */
   public IntegerSketch(final int lgK, final IntegerSummary.Mode mode) {
-    super(1 << lgK, ResizeFactor.X8.ordinal(), 1.0F, new IntegerSummaryFactory(mode));
+    this(lgK, ResizeFactor.X8.ordinal(), 1.0F, mode);
+  }
+
+  /**
+   * Creates this sketch with the following parameters:
+   * @param lgK Log_base2 of <i>Nominal Entries</i>.
+   * @param lgResizeFactor log2(resizeFactor) - value from 0 to 3:
+   * <pre>
+   * 0 - no resizing (max size allocated),
+   * 1 - double internal hash table each time it reaches a threshold
+   * 2 - grow four times
+   * 3 - grow eight times (default)
+   * </pre>
+   * @param samplingProbability
+   * <a href="{@docRoot}/resources/dictionary.html#p">See Sampling Probability</a>
+   * @param mode The IntegerSummary mode to be used
+   */
+  public IntegerSketch(final int lgK, final int lgResizeFactor, final float samplingProbability,
+      final IntegerSummary.Mode mode) {
+    super(1 << lgK, lgResizeFactor, samplingProbability, new IntegerSummaryFactory(mode));
   }
 
   /**
diff --git a/src/test/java/org/apache/datasketches/tuple/adouble/UpdatableSketchWithDoubleSummaryTest.java b/src/test/java/org/apache/datasketches/tuple/adouble/UpdatableSketchWithDoubleSummaryTest.java
index 8e1aefac7..cfc4dcf71 100644
--- a/src/test/java/org/apache/datasketches/tuple/adouble/UpdatableSketchWithDoubleSummaryTest.java
+++ b/src/test/java/org/apache/datasketches/tuple/adouble/UpdatableSketchWithDoubleSummaryTest.java
@@ -230,13 +230,6 @@ public void updatesOfAllKeyTypes() {
     Assert.assertEquals(sketch.getEstimate(), 6.0);
   }
 
-//  @Test
-//  public void updateDoubleSummary() {
-//    DoubleSummary ds = new DoubleSummary();
-//    ds.update(1.0);
-//    Assert.assertEquals(ds.getValue(), 1.0);
-//  }
-
   @Test
   public void doubleSummaryDefaultSumMode() {
     UpdatableSketch<Double, DoubleSummary> sketch =
@@ -402,6 +395,22 @@ public void serializeDeserializeSampling() throws Exception {
     Assert.assertEquals(sketch1.getTheta(), sketch2.getTheta());
   }
 
+  @Test
+  public void unionEmptySampling() {
+    UpdatableSketch<Double, DoubleSummary> sketch =
+        new UpdatableSketchBuilder<>(new DoubleSummaryFactory(mode)).setSamplingProbability(0.01f).build();
+    sketch.update(1, 1.0);
+    Assert.assertEquals(sketch.getRetainedEntries(), 0); // not retained due to low sampling probability
+
+    Union<DoubleSummary> union = new Union<>(new DoubleSummarySetOperations(mode));
+    union.update(sketch);
+    CompactSketch<DoubleSummary> result = union.getResult();
+    Assert.assertEquals(result.getRetainedEntries(), 0);
+    Assert.assertFalse(result.isEmpty());
+    Assert.assertTrue(result.isEstimationMode());
+    Assert.assertEquals(result.getEstimate(), 0.0);
+  }
+
   @Test
   public void unionExactMode() {
     UpdatableSketch<Double, DoubleSummary> sketch1 =