Add skewness Spark agg function

velox/docs/functions/spark/aggregate.rst

@@ -103,6 +103,12 @@ General Aggregate Functions
 
     Returns b
 
+.. spark:function:: skewness(x) -> double
+
+    Returns the skewness of all input values. When the count of `x` is greater than or equal to 1,
+    a non-null output will be generated. When the value of `m2` in the accumulator is 0, a null
+    output will be generated.
+
 .. spark:function:: sum(x) -> bigint|double|real
 
     Returns the sum of `x`.

velox/functions/lib/aggregates/CMakeLists.txt

@@ -12,8 +12,10 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 
-add_library(velox_functions_aggregates SingleValueAccumulator.cpp
-                                       AverageAggregateBase.cpp ValueSet.cpp)
+add_library(
+  velox_functions_aggregates
+  AverageAggregateBase.cpp CentralMomentsAggregatesBase.cpp
+  SingleValueAccumulator.cpp ValueSet.cpp)
 
 target_link_libraries(velox_functions_aggregates velox_exec
                       velox_presto_serializer Folly::folly)

velox/functions/lib/aggregates/CentralMomentsAggregatesBase.cpp

@@ -0,0 +1,52 @@
+/*
+ * Copyright (c) Facebook, Inc. and its affiliates.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+#include "velox/functions/lib/aggregates/CentralMomentsAggregatesBase.h"
+
+namespace facebook::velox::functions::aggregate {
+
+void checkAccumulatorRowType(
+    const TypePtr& type,
+    const std::string& errorMessage) {
+  VELOX_CHECK_EQ(type->kind(), TypeKind::ROW, "{}", errorMessage);
+  VELOX_CHECK_EQ(
+      type->childAt(kCentralMomentsIndices.count)->kind(),
+      TypeKind::BIGINT,
+      "{}",
+      errorMessage);
+  VELOX_CHECK_EQ(
+      type->childAt(kCentralMomentsIndices.m1)->kind(),
+      TypeKind::DOUBLE,
+      "{}",
+      errorMessage);
+  VELOX_CHECK_EQ(
+      type->childAt(kCentralMomentsIndices.m2)->kind(),
+      TypeKind::DOUBLE,
+      "{}",
+      errorMessage);
+  VELOX_CHECK_EQ(
+      type->childAt(kCentralMomentsIndices.m3)->kind(),
+      TypeKind::DOUBLE,
+      "{}",
+      errorMessage);
+  VELOX_CHECK_EQ(
+      type->childAt(kCentralMomentsIndices.m4)->kind(),
+      TypeKind::DOUBLE,
+      "{}",
+      errorMessage);
+}
+
+} // namespace facebook::velox::functions::aggregate