handle empty sketches (#7526)

* handle empty sketches

* return array of NaN in case of empty sketch

* noinspection ForLoopReplaceableByForEach in tests

* style fixes

extensions-core/datasketches/src/main/java/org/apache/druid/query/aggregation/datasketches/quantiles/DoublesSketchToHistogramPostAggregator.java

@@ -56,6 +56,11 @@ public class DoublesSketchToHistogramPostAggregator implements PostAggregator
   public Object compute(final Map<String, Object> combinedAggregators)
   {
     final DoublesSketch sketch = (DoublesSketch) field.compute(combinedAggregators);
+    if (sketch.isEmpty()) {
+      final double[] histogram = new double[splitPoints.length + 1];
+      Arrays.fill(histogram, Double.NaN);
+      return histogram;
+    }
     final double[] histogram = sketch.getPMF(splitPoints);
     for (int i = 0; i < histogram.length; i++) {
       histogram[i] *= sketch.getN();

extensions-core/datasketches/src/main/java/org/apache/druid/query/aggregation/datasketches/quantiles/DoublesSketchToQuantilesPostAggregator.java

@@ -75,6 +75,11 @@ public class DoublesSketchToQuantilesPostAggregator implements PostAggregator
   public Object compute(final Map<String, Object> combinedAggregators)
   {
     final DoublesSketch sketch = (DoublesSketch) field.compute(combinedAggregators);
+    if (sketch.isEmpty()) {
+      final double[] quantiles = new double[fractions.length];
+      Arrays.fill(quantiles, Double.NaN);
+      return quantiles;
+    }
     return sketch.getQuantiles(fractions);
   }
 

extensions-core/datasketches/src/test/java/org/apache/druid/query/aggregation/datasketches/quantiles/DoublesSketchToHistogramPostAggregatorTest.java

@@ -0,0 +1,84 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one
+ * or more contributor license agreements.  See the NOTICE file
+ * distributed with this work for additional information
+ * regarding copyright ownership.  The ASF licenses this file
+ * to you under the Apache License, Version 2.0 (the
+ * "License"); you may not use this file except in compliance
+ * with the License.  You may obtain a copy of the License at
+ *
+ *   http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing,
+ * software distributed under the License is distributed on an
+ * "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+ * KIND, either express or implied.  See the License for the
+ * specific language governing permissions and limitations
+ * under the License.
+ */
+
+package org.apache.druid.query.aggregation.datasketches.quantiles;
+
+import org.apache.druid.query.aggregation.Aggregator;
+import org.apache.druid.query.aggregation.PostAggregator;
+import org.apache.druid.query.aggregation.TestDoubleColumnSelectorImpl;
+import org.apache.druid.query.aggregation.post.FieldAccessPostAggregator;
+import org.junit.Assert;
+import org.junit.Test;
+
+import java.util.HashMap;
+import java.util.Map;
+
+public class DoublesSketchToHistogramPostAggregatorTest
+{
+  @Test
+  public void emptySketch()
+  {
+    final TestDoubleColumnSelectorImpl selector = new TestDoubleColumnSelectorImpl(null);
+    final Aggregator agg = new DoublesSketchBuildAggregator(selector, 8);
+
+    final Map<String, Object> fields = new HashMap<>();
+    fields.put("sketch", agg.get());
+
+    final PostAggregator postAgg = new DoublesSketchToHistogramPostAggregator(
+        "histogram",
+        new FieldAccessPostAggregator("field", "sketch"),
+        new double[] {3.5}
+    );
+
+    final double[] histogram = (double[]) postAgg.compute(fields);
+    Assert.assertNotNull(histogram);
+    Assert.assertEquals(2, histogram.length);
+    Assert.assertTrue(Double.isNaN(histogram[0]));
+    Assert.assertTrue(Double.isNaN(histogram[1]));
+  }
+
+  @Test
+  public void normalCase()
+  {
+    final double[] values = new double[] {1, 2, 3, 4, 5, 6};
+    final TestDoubleColumnSelectorImpl selector = new TestDoubleColumnSelectorImpl(values);
+
+    final Aggregator agg = new DoublesSketchBuildAggregator(selector, 8);
+    //noinspection ForLoopReplaceableByForEach
+    for (int i = 0; i < values.length; i++) {
+      agg.aggregate();
+      selector.increment();
+    }
+
+    final Map<String, Object> fields = new HashMap<>();
+    fields.put("sketch", agg.get());
+
+    final PostAggregator postAgg = new DoublesSketchToHistogramPostAggregator(
+        "histogram",
+        new FieldAccessPostAggregator("field", "sketch"),
+        new double[] {3.5} // splits distribution in two buckets of equal mass 
+    );
+
+    final double[] histogram = (double[]) postAgg.compute(fields);
+    Assert.assertNotNull(histogram);
+    Assert.assertEquals(2, histogram.length);
+    Assert.assertEquals(3.0, histogram[0], 0);
+    Assert.assertEquals(3.0, histogram[1], 0);
+  }
+}

extensions-core/datasketches/src/test/java/org/apache/druid/query/aggregation/datasketches/quantiles/DoublesSketchToQuantilesPostAggregatorTest.java

@@ -0,0 +1,86 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one
+ * or more contributor license agreements.  See the NOTICE file
+ * distributed with this work for additional information
+ * regarding copyright ownership.  The ASF licenses this file
+ * to you under the Apache License, Version 2.0 (the
+ * "License"); you may not use this file except in compliance
+ * with the License.  You may obtain a copy of the License at
+ *
+ *   http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing,
+ * software distributed under the License is distributed on an
+ * "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+ * KIND, either express or implied.  See the License for the
+ * specific language governing permissions and limitations
+ * under the License.
+ */
+
+package org.apache.druid.query.aggregation.datasketches.quantiles;
+
+import org.apache.druid.query.aggregation.Aggregator;
+import org.apache.druid.query.aggregation.PostAggregator;
+import org.apache.druid.query.aggregation.TestDoubleColumnSelectorImpl;
+import org.apache.druid.query.aggregation.post.FieldAccessPostAggregator;
+import org.junit.Assert;
+import org.junit.Test;
+
+import java.util.HashMap;
+import java.util.Map;
+
+public class DoublesSketchToQuantilesPostAggregatorTest
+{
+  @Test
+  public void emptySketch()
+  {
+    final TestDoubleColumnSelectorImpl selector = new TestDoubleColumnSelectorImpl(null);
+    final Aggregator agg = new DoublesSketchBuildAggregator(selector, 8);
+
+    final Map<String, Object> fields = new HashMap<>();
+    fields.put("sketch", agg.get());
+
+    final PostAggregator postAgg = new DoublesSketchToQuantilesPostAggregator(
+        "quantiles",
+        new FieldAccessPostAggregator("field", "sketch"),
+        new double[] {0, 0.5, 1}
+    );
+
+    final double[] quantiles = (double[]) postAgg.compute(fields);
+    Assert.assertNotNull(quantiles);
+    Assert.assertEquals(3, quantiles.length);
+    Assert.assertTrue(Double.isNaN(quantiles[0]));
+    Assert.assertTrue(Double.isNaN(quantiles[1]));
+    Assert.assertTrue(Double.isNaN(quantiles[2]));
+  }
+
+  @Test
+  public void normalCase()
+  {
+    final double[] values = new double[] {1, 2, 3, 4, 5};
+    final TestDoubleColumnSelectorImpl selector = new TestDoubleColumnSelectorImpl(values);
+
+    final Aggregator agg = new DoublesSketchBuildAggregator(selector, 8);
+    //noinspection ForLoopReplaceableByForEach
+    for (int i = 0; i < values.length; i++) {
+      agg.aggregate();
+      selector.increment();
+    }
+
+    final Map<String, Object> fields = new HashMap<>();
+    fields.put("sketch", agg.get());
+
+    final PostAggregator postAgg = new DoublesSketchToQuantilesPostAggregator(
+        "quantiles",
+        new FieldAccessPostAggregator("field", "sketch"),
+        new double[] {0, 0.5, 1}
+    );
+
+    final double[] quantiles = (double[]) postAgg.compute(fields);
+    Assert.assertNotNull(quantiles);
+    Assert.assertEquals(3, quantiles.length);
+    Assert.assertEquals(1.0, quantiles[0], 0);
+    Assert.assertEquals(3.0, quantiles[1], 0);
+    Assert.assertEquals(5.0, quantiles[2], 0);
+  }
+}