Refactor skewness,corr calculation in central moment accumulators

Author: Cool6689

iotdb-core/datanode/src/main/java/org/apache/iotdb/db/queryengine/execution/aggregation/CentralMomentAccumulator.java

@@ -177,13 +177,10 @@ public void outputFinal(ColumnBuilder columnBuilder) {
     if (momentType == MomentType.SKEWNESS) {
       if (count < 3) {
         columnBuilder.appendNull();
-      } else {
-
-        double variance = m2 / (count - 1);
-        double stdev = Math.sqrt(variance);
-        double result = (count * m3) / ((count - 1) * (count - 2) * stdev * stdev * stdev);
-        columnBuilder.writeDouble(result);
+        return;
       }
+      double result = Math.sqrt((double) count) * m3 / Math.pow(m2, 1.5);
+      columnBuilder.writeDouble(result);
     } else {
       if (count < 4) {
         columnBuilder.appendNull();

iotdb-core/datanode/src/main/java/org/apache/iotdb/db/queryengine/execution/aggregation/CorrelationAccumulator.java

@@ -180,7 +180,7 @@ public void outputFinal(ColumnBuilder columnBuilder) {
           columnBuilder.appendNull();
         } else if (m2X == 0 || m2Y == 0) {
 
-          columnBuilder.writeDouble(0.0);
+          columnBuilder.appendNull();
         } else {
           columnBuilder.writeDouble(c2 / Math.sqrt(m2X * m2Y));
         }

iotdb-core/datanode/src/main/java/org/apache/iotdb/db/queryengine/execution/operator/source/relational/aggregation/TableCentralMomentAccumulator.java

@@ -192,12 +192,10 @@ public void evaluateFinal(ColumnBuilder columnBuilder) {
     if (momentType == CentralMomentAccumulator.MomentType.SKEWNESS) {
       if (count < 3) {
         columnBuilder.appendNull();
-      } else {
-        double variance = m2 / (count - 1);
-        double stdev = Math.sqrt(variance);
-        double result = (count * m3) / ((count - 1) * (count - 2) * stdev * stdev * stdev);
-        columnBuilder.writeDouble(result);
+        return;
       }
+      double result = Math.sqrt((double) count) * m3 / Math.pow(m2, 1.5);
+      columnBuilder.writeDouble(result);
     } else {
       if (count < 4) {
         columnBuilder.appendNull();

iotdb-core/datanode/src/main/java/org/apache/iotdb/db/queryengine/execution/operator/source/relational/aggregation/TableCorrelationAccumulator.java

@@ -216,7 +216,7 @@ public void evaluateFinal(ColumnBuilder columnBuilder) {
         if (count < 2) {
           columnBuilder.appendNull();
         } else if (m2X == 0 || m2Y == 0) {
-          columnBuilder.writeDouble(0.0);
+          columnBuilder.appendNull();
         } else {
           columnBuilder.writeDouble(c2 / Math.sqrt(m2X * m2Y));
         }

iotdb-core/datanode/src/main/java/org/apache/iotdb/db/queryengine/execution/operator/source/relational/aggregation/grouped/GroupedCentralMomentAccumulator.java

@@ -242,13 +242,11 @@ public void evaluateFinal(int groupId, ColumnBuilder columnBuilder) {
     if (momentType == CentralMomentAccumulator.MomentType.SKEWNESS) {
       if (count < 3) {
         columnBuilder.appendNull();
-      } else {
-        double m3 = m3s.get(groupId);
-        double variance = m2 / (count - 1);
-        double stdev = Math.sqrt(variance);
-        double result = (count * m3) / ((count - 1) * (count - 2) * stdev * stdev * stdev);
-        columnBuilder.writeDouble(result);
+        return;
       }
+      double m3 = m3s.get(groupId);
+      double result = Math.sqrt((double) count) * m3 / Math.pow(m2, 1.5);
+      columnBuilder.writeDouble(result);
     } else {
       if (count < 4) {
         columnBuilder.appendNull();

iotdb-core/datanode/src/main/java/org/apache/iotdb/db/queryengine/execution/operator/source/relational/aggregation/grouped/GroupedCorrelationAccumulator.java

@@ -230,7 +230,7 @@ public void evaluateFinal(int groupId, ColumnBuilder columnBuilder) {
         if (counts.get(groupId) < 2) {
           columnBuilder.appendNull();
         } else if (m2Xs.get(groupId) == 0 || m2Ys.get(groupId) == 0) {
-          columnBuilder.writeDouble(0.0);
+          columnBuilder.appendNull();
         } else {
           columnBuilder.writeDouble(
               c2s.get(groupId) / Math.sqrt(m2Xs.get(groupId) * m2Ys.get(groupId)));