[CH] Support skewness aggregate function

backends-clickhouse/src/main/scala/org/apache/gluten/utils/CHExpressionUtil.scala

@@ -192,7 +192,6 @@ object CHExpressionUtil {
     URL_DECODE -> DefaultValidator(),
     URL_ENCODE -> DefaultValidator(),
     FORMAT_STRING -> FormatStringValidator(),
-    SKEWNESS -> DefaultValidator(),
     MAKE_YM_INTERVAL -> DefaultValidator(),
     MAP_ZIP_WITH -> DefaultValidator(),
     KURTOSIS -> DefaultValidator(),

backends-clickhouse/src/test/scala/org/apache/gluten/execution/GlutenClickhouseCountDistinctSuite.scala

@@ -100,12 +100,14 @@ class GlutenClickhouseCountDistinctSuite extends GlutenClickHouseWholeStageTrans
   }
 
   test("check count distinct with agg fallback") {
-    // skewness agg is not supported, will cause fallback
     val sql = "select count(distinct(a,b)) , skewness(b) from " +
       "values (0, null,1), (0,null,1), (1, 1,1), (2, 2, 1) ,(2,2,2),(3,3,3) as data(a,b,c)"
-    assertThrows[UnsupportedOperationException] {
-      spark.sql(sql).show
-    }
+    compareResultsAgainstVanillaSpark(sql, true, { _ => })
+
+    val sqlWithKeys = "select a, count(distinct(b)) , skewness(b) from " +
+      "values (0, null,1), (0,null,1), (1, 1,1), (2, 2, 1) ,(2,2,2),(3,3,3) as data(a,b,c) " +
+      "group by a"
+    compareResultsAgainstVanillaSpark(sqlWithKeys, true, { _ => })
   }
 
   test("check count distinct with expr fallback") {

cpp-ch/local-engine/Parser/aggregate_function_parser/SimpleStatisticsFunctions.cpp

@@ -63,7 +63,14 @@ class AggregateFunctionParserStddev final : public AggregateFunctionParser
         return func_node;
     }
 };
+// for skewness
+struct SkewnessNameStruct
+{
+    static constexpr auto spark_name = "skewness";
+    static constexpr auto ch_name = "skewSamp";
+};
 
+static const AggregateFunctionParserRegister<AggregateFunctionParserStddev<SkewnessNameStruct>> registerer_skewness;
 static const AggregateFunctionParserRegister<AggregateFunctionParserStddev<StddevNameStruct>> registerer_stddev;
 static const AggregateFunctionParserRegister<AggregateFunctionParserStddev<StddevSampNameStruct>> registerer_stddev_samp;
 }