[ML] Fix r_squared eval when variance is 0 (#49439) (#49445)

Author: benwtrent

x-pack/plugin/core/src/main/java/org/elasticsearch/xpack/core/ml/dataframe/evaluation/regression/RSquared.java

@@ -81,7 +81,11 @@ public void process(Aggregations aggs) {
         NumericMetricsAggregation.SingleValue residualSumOfSquares = aggs.get(SS_RES);
         ExtendedStats extendedStats = aggs.get(ExtendedStatsAggregationBuilder.NAME + "_actual");
         // extendedStats.getVariance() is the statistical sumOfSquares divided by count
-        result = residualSumOfSquares == null || extendedStats == null || extendedStats.getCount() == 0 ?
+        final boolean validResult = residualSumOfSquares == null
+            || extendedStats == null
+            || extendedStats.getCount() == 0
+            || extendedStats.getVariance() == 0;
+        result = validResult ?
             new Result(0.0) :
             new Result(1 - (residualSumOfSquares.value() / (extendedStats.getVariance() * extendedStats.getCount())));
     }

x-pack/plugin/core/src/test/java/org/elasticsearch/xpack/core/ml/dataframe/evaluation/regression/RSquaredTests.java

@@ -74,6 +74,21 @@ public void testEvaluateWithZeroCount() {
         assertThat(result, equalTo(new RSquared.Result(0.0)));
     }
 
+    public void testEvaluateWithSingleCountZeroVariance() {
+        Aggregations aggs = new Aggregations(Arrays.asList(
+            createSingleMetricAgg("residual_sum_of_squares", 1),
+            createExtendedStatsAgg("extended_stats_actual", 0.0, 1),
+            createExtendedStatsAgg("some_other_extended_stats",99.1, 10_000),
+            createSingleMetricAgg("some_other_single_metric_agg", 0.2377)
+        ));
+
+        RSquared rSquared = new RSquared();
+        rSquared.process(aggs);
+
+        EvaluationMetricResult result = rSquared.getResult().get();
+        assertThat(result, equalTo(new RSquared.Result(0.0)));
+    }
+
     public void testEvaluate_GivenMissingAggs() {
         Aggregations aggs = new Aggregations(Collections.singletonList(
             createSingleMetricAgg("some_other_single_metric_agg", 0.2377)