Refactor skewness calculation in central moment accumulators

Author: Cool6689

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

@@ -175,15 +175,8 @@ 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);
-      }
+      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/TableCentralMomentAccumulator.java

@@ -190,14 +190,8 @@ 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);
-      }
+      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/GroupedCentralMomentAccumulator.java

@@ -240,15 +240,9 @@ 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);
-      }
+      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();