Serialize sum as null instead of 0.0 for empty histograms.

Author: JonasKunz

libs/exponential-histogram/src/main/java/org/elasticsearch/exponentialhistogram/ExponentialHistogramXContent.java

@@ -63,7 +63,10 @@ public static void serialize(XContentBuilder builder, @Nullable ExponentialHisto
         builder.startObject();
 
         builder.field(SCALE_FIELD, histogram.scale());
-        builder.field(SUM_FIELD, histogram.sum());
+
+        if (histogram.sum() != 0.0 || histogram.valueCount() > 0) {
+            builder.field(SUM_FIELD, histogram.sum());
+        }
         if (Double.isNaN(histogram.min()) == false) {
             builder.field(MIN_FIELD, histogram.min());
         }

libs/exponential-histogram/src/test/java/org/elasticsearch/exponentialhistogram/ExponentialHistogramXContentTests.java

@@ -40,7 +40,7 @@ public void testNullHistogram() {
 
     public void testEmptyHistogram() {
         ExponentialHistogram emptyHistogram = ExponentialHistogram.empty();
-        assertThat(toJson(emptyHistogram), equalTo("{\"scale\":" + emptyHistogram.scale() + ",\"sum\":0.0}"));
+        assertThat(toJson(emptyHistogram), equalTo("{\"scale\":" + emptyHistogram.scale() + "}"));
     }
 
     public void testFullHistogram() {

x-pack/plugin/esql/qa/testFixtures/src/main/resources/exponential_histogram.csv-spec

@@ -5,13 +5,13 @@ FROM exp_histo_sample | WHERE STARTS_WITH(instance, "dummy") | SORT instance | K
 ;
 
 instance:keyword          | responseTime:exponential_histogram
-dummy-empty               | "{""scale"":-7,""sum"":0.0}"
+dummy-empty               | "{""scale"":-7}"
 dummy-full                | "{""scale"":0,""sum"":-3775.0,""min"":-100.0,""max"":50.0,""zero"":{""count"":1,""threshold"":1.0E-4},""positive"":{""indices"":[-1,0,1,2,3,4,5],""counts"":[1,1,2,4,8,16,18]},""negative"":{""indices"":[-1,0,1,2,3,4,5,6],""counts"":[1,1,2,4,8,16,32,36]}}"
 dummy-negative_only       | "{""scale"":2,""sum"":-1275.0,""min"":-50.0,""max"":-1.0,""negative"":{""indices"":[-1,3,6,7,9,10,11,12,13,14,15,16,17,18,19,20,21,22],""counts"":[1,1,1,1,1,1,2,1,2,2,3,3,3,4,6,6,7,5]}}"
 dummy-no_zero_bucket      | "{""scale"":0,""sum"":-3775.0,""min"":-100.0,""max"":50.0,""positive"":{""indices"":[-1,0,1,2,3,4,5],""counts"":[1,1,2,4,8,16,18]},""negative"":{""indices"":[-1,0,1,2,3,4,5,6],""counts"":[1,1,2,4,8,16,32,36]}}"
 dummy-positive_only       | "{""scale"":2,""sum"":1275.0,""min"":1.0,""max"":50.0,""positive"":{""indices"":[-1,3,6,7,9,10,11,12,13,14,15,16,17,18,19,20,21,22],""counts"":[1,1,1,1,1,1,2,1,2,2,3,3,3,4,6,6,7,5]}}"
 dummy-zero_count_only     | "{""scale"":2,""sum"":0.0,""min"":0.0,""max"":0.0,""zero"":{""count"":101}}"
-dummy-zero_threshold_only | "{""scale"":0,""sum"":0.0,""zero"":{""threshold"":2.0E-5}}"
+dummy-zero_threshold_only | "{""scale"":0,""zero"":{""threshold"":2.0E-5}}"
 ;
 
 

x-pack/plugin/mapper-exponential-histogram/src/test/java/org/elasticsearch/xpack/exponentialhistogram/ExponentialHistogramFieldMapperTests.java

@@ -568,21 +568,30 @@ private Map<String, Object> convertHistogramToCanonicalForm(Map<String, Object>
 
                 Map<String, Object> zeroBucket = convertZeroBucketToCanonicalForm(Types.forciblyCast(histogram.get("zero")));
 
-                Object sum = histogram.get("sum");
-                if (sum == null) {
-                    sum = ExponentialHistogramUtils.estimateSum(
-                        IndexWithCount.asBuckets(scale, negative).iterator(),
-                        IndexWithCount.asBuckets(scale, positive).iterator()
-                    );
+                Number sum = (Number) histogram.get("sum");
+                ExponentialHistogram.Buckets negativeBuckets = IndexWithCount.asBuckets(scale, negative);
+                ExponentialHistogram.Buckets positiveBuckets = IndexWithCount.asBuckets(scale, positive);
+
+                boolean isEmpty = negativeBuckets.iterator().hasNext() == false
+                    && positiveBuckets.iterator().hasNext() == false
+                    && (zeroBucket == null || Types.<Number>forciblyCast(zeroBucket.getOrDefault("count", 0L)).longValue() == 0L);
+
+                // we allow 0.0 as sum for input histograms, but output null in canonical form in that case
+                if (isEmpty && (sum == null || sum.doubleValue() == 0.0)) {
+                    sum = null;
+                } else if (sum == null) {
+                    sum = ExponentialHistogramUtils.estimateSum(negativeBuckets.iterator(), positiveBuckets.iterator());
+                }
+                if (sum != null) {
+                    result.put("sum", sum);
                 }
-                result.put("sum", sum);
 
                 Object min = histogram.get("min");
                 if (min == null) {
                     OptionalDouble estimatedMin = ExponentialHistogramUtils.estimateMin(
                         mapToZeroBucket(zeroBucket),
-                        IndexWithCount.asBuckets(scale, negative),
-                        IndexWithCount.asBuckets(scale, positive)
+                        negativeBuckets,
+                        positiveBuckets
                     );
                     if (estimatedMin.isPresent()) {
                         min = estimatedMin.getAsDouble();
@@ -596,8 +605,8 @@ private Map<String, Object> convertHistogramToCanonicalForm(Map<String, Object>
                 if (max == null) {
                     OptionalDouble estimatedMax = ExponentialHistogramUtils.estimateMax(
                         mapToZeroBucket(zeroBucket),
-                        IndexWithCount.asBuckets(scale, negative),
-                        IndexWithCount.asBuckets(scale, positive)
+                        negativeBuckets,
+                        positiveBuckets
                     );
                     if (estimatedMax.isPresent()) {
                         max = estimatedMax.getAsDouble();

x-pack/plugin/mapper-exponential-histogram/src/yamlRestTest/resources/rest-api-spec/test/10_synthetic_source.yml

@@ -161,6 +161,24 @@ setup:
           counts: [3, 2, 1]
 ---
 "Empty histogram":
+  - do:
+      index:
+        index: test_exponential_histogram
+        id: "1"
+        refresh: true
+        body:
+          my_histo:
+            scale: -7
+  - do:
+      get:
+        index: test_exponential_histogram
+        id: "1"
+  - match:
+      _source.my_histo:
+        scale: -7
+
+---
+"Empty histogram with sum":
   - do:
       index:
         index: test_exponential_histogram
@@ -176,7 +194,6 @@ setup:
         id: "1"
   - match:
       _source.my_histo:
-        sum: 0.0
         scale: -7
 
 ---
@@ -198,7 +215,6 @@ setup:
   - match:
       _source.my_histo:
         scale: 0
-        sum: 0.0
         zero:
           threshold: 42.7