Reset sketch combiner in AggregatorCombiner

extensions-core/datasketches/src/main/java/org/apache/druid/query/aggregation/datasketches/theta/SketchAggregatorFactory.java

@@ -107,6 +107,7 @@ public AggregateCombiner makeAggregateCombiner()
       public void reset(ColumnValueSelector selector)
       {
         union.reset();
+        combined.invalidateCache();
         fold(selector);
       }
 

extensions-core/datasketches/src/test/java/org/apache/druid/query/aggregation/datasketches/theta/SketchAggregationTest.java

@@ -146,6 +146,41 @@ public void testSketchDataIngestAndGpByQuery() throws Exception
     );
   }
 
+  @Test
+  public void testEmptySketchAggregateCombine() throws Exception
+  {
+    final String groupByQueryString = readFileFromClasspathAsString("empty_sketch_group_by_query.json");
+    final GroupByQuery groupByQuery = (GroupByQuery) helper.getObjectMapper()
+                                                           .readValue(groupByQueryString, Query.class);
+
+    final Sequence<ResultRow> seq = helper.createIndexAndRunQueryOnSegment(
+        new File(SketchAggregationTest.class.getClassLoader().getResource("empty_sketch_data.tsv").getFile()),
+        readFileFromClasspathAsString("empty_sketch_data_record_parser.json"),
+        readFileFromClasspathAsString("empty_sketch_test_data_aggregators.json"),
+        0,
+        Granularities.NONE,
+        5,
+        groupByQueryString
+    );
+
+    List<ResultRow> results = seq.toList();
+    Assert.assertEquals(1, results.size());
+    Assert.assertEquals(
+        ResultRow.fromLegacyRow(
+            new MapBasedRow(
+                DateTimes.of("2019-07-14T00:00:00.000Z"),
+                ImmutableMap
+                    .<String, Object>builder()
+                    .put("product", "product_b")
+                    .put("sketch_count", 0.0)
+                    .build()
+            ),
+            groupByQuery
+        ),
+        results.get(0)
+    );
+  }
+
   @Test
   public void testThetaCardinalityOnSimpleColumn() throws Exception
   {

extensions-core/datasketches/src/test/resources/empty_sketch_data.tsv

@@ -0,0 +1,16 @@
+2019071401	product_c	LN	AwEDAAAAAgABAAAAAAAAAP////////9/RI5olqYUtnQ=
+2019071401	product_c	LN	AwEDAAAAAgABAAAAAAAAAP////////9/EOmCmVerjm4=
+2019071401	product_d	ZN	AQMDAAAezJM=
+2019071401	product_a	DN	AwEDAAAAAgABAAAAAAAAAP////////9/i3zstpLhWWs=
+2019071401	product_b	ZN	AQMDAAAezJM=
+2019071401	product_a	CN	AwEDAAAAAgABAAAAAAAAAP////////9/o31ldGC0E2s=
+2019071401	product_a	GN	AwEDAAAAAgABAAAAAAAAAP////////9/i3zstpLhWWs=
+2019071401	product_b	ZN	AQMDAAAezJM=
+2019071401	product_a	LN	AwEDAAAAAgABAAAAAAAAAP////////9/KKcfz0hRe38=
+2019071401	product_a	SN	AwEDAAAAAgABAAAAAAAAAP////////9/jDwgknTL/S0=
+2019071401	product_d	LN	AwEDAAAAAgABAAAAAAAAAP////////9/i3zstpLhWWs=
+2019071401	product_a	SN	AwEDAAAAAgABAAAAAAAAAP////////9/4EyBvXLM/xs=
+2019071401	product_a	LN	AwEDAAAAAgABAAAAAAAAAP////////9/KKcfz0hRe38=
+2019071401	product_d	ZN	AQMDAAAezJM=
+2019071401	product_a	ZN	AwEDAAAAAgABAAAAAAAAAP////////9//mseeN0UsgU=
+2019071401	product_c	LN	AwEDAAAAAgABAAAAAAAAAP////////9/Rn5CRuhP3h4=

extensions-core/datasketches/src/test/resources/empty_sketch_data_record_parser.json

@@ -0,0 +1,24 @@
+{
+  "type": "string",
+  "parseSpec": {
+    "format": "tsv",
+    "timestampSpec": {
+      "column": "timestamp",
+      "format": "yyyyMMddHH"
+    },
+    "dimensionsSpec": {
+      "dimensions": [
+        "product",
+        "product_code"
+      ],
+      "dimensionExclusions": [],
+      "spatialDimensions": []
+    },
+    "columns": [
+      "timestamp",
+      "product",
+      "product_code",
+      "product_sketch"
+    ]
+  }
+}

extensions-core/datasketches/src/test/resources/empty_sketch_group_by_query.json

@@ -0,0 +1,20 @@
+{
+  "queryType": "groupBy",
+  "dataSource": "test_datasource",
+  "granularity":"ALL",
+  "dimensions": ["product"],
+  "filter" : {
+    "type" : "selector", "dimension" : "product", "value" : "product_b"
+  },
+  "aggregations": [
+    {
+      "type": "thetaSketch",
+      "name": "sketch_count",
+      "fieldName": "product_sketch",
+      "size": 16384
+    }
+  ],
+  "intervals": [
+    "2019-07-14T00:00:00.000Z/2019-07-15T00:00:00.000Z"
+  ]
+}

extensions-core/datasketches/src/test/resources/empty_sketch_test_data_aggregators.json

@@ -0,0 +1,9 @@
+[
+  {
+    "type": "thetaSketch",
+    "name": "product_sketch",
+    "fieldName": "product_sketch",
+    "isInputThetaSketch": true,
+    "size": 16384
+  }
+]