leanken commented on a change in pull request #29983:
URL: https://github.com/apache/spark/pull/29983#discussion_r503069104



##########
File path: 
sql/hive/src/test/scala/org/apache/spark/sql/hive/execution/WindowQuerySuite.scala
##########
@@ -59,56 +60,115 @@ class WindowQuerySuite extends QueryTest with SQLTestUtils 
with TestHiveSingleto
   }
 
   test("windowing.q -- 15. testExpressions") {
-    // Moved because:
-    // - Spark uses a different default stddev (sample instead of pop)
-    // - Tiny numerical differences in stddev results.
-    // - Different StdDev behavior when n=1 (NaN instead of 0)
-    checkAnswer(sql(s"""
-      |select  p_mfgr,p_name, p_size,
-      |rank() over(distribute by p_mfgr sort by p_name) as r,
-      |dense_rank() over(distribute by p_mfgr sort by p_name) as dr,
-      |cume_dist() over(distribute by p_mfgr sort by p_name) as cud,
-      |percent_rank() over(distribute by p_mfgr sort by p_name) as pr,
-      |ntile(3) over(distribute by p_mfgr sort by p_name) as nt,
-      |count(p_size) over(distribute by p_mfgr sort by p_name) as ca,
-      |avg(p_size) over(distribute by p_mfgr sort by p_name) as avg,
-      |stddev(p_size) over(distribute by p_mfgr sort by p_name) as st,
-      |first_value(p_size % 5) over(distribute by p_mfgr sort by p_name) as fv,
-      |last_value(p_size) over(distribute by p_mfgr sort by p_name) as lv,
-      |first_value(p_size) over w1  as fvW1
-      |from part
-      |window w1 as (distribute by p_mfgr sort by p_mfgr, p_name
-      |             rows between 2 preceding and 2 following)
+    withSQLConf(SQLConf.LEGACY_CENTRAL_MOMENT_AGG.key -> "true") {

Review comment:
       done.

##########
File path: 
sql/hive/src/test/scala/org/apache/spark/sql/hive/execution/WindowQuerySuite.scala
##########
@@ -59,56 +60,115 @@ class WindowQuerySuite extends QueryTest with SQLTestUtils 
with TestHiveSingleto
   }
 
   test("windowing.q -- 15. testExpressions") {
-    // Moved because:
-    // - Spark uses a different default stddev (sample instead of pop)
-    // - Tiny numerical differences in stddev results.
-    // - Different StdDev behavior when n=1 (NaN instead of 0)
-    checkAnswer(sql(s"""
-      |select  p_mfgr,p_name, p_size,
-      |rank() over(distribute by p_mfgr sort by p_name) as r,
-      |dense_rank() over(distribute by p_mfgr sort by p_name) as dr,
-      |cume_dist() over(distribute by p_mfgr sort by p_name) as cud,
-      |percent_rank() over(distribute by p_mfgr sort by p_name) as pr,
-      |ntile(3) over(distribute by p_mfgr sort by p_name) as nt,
-      |count(p_size) over(distribute by p_mfgr sort by p_name) as ca,
-      |avg(p_size) over(distribute by p_mfgr sort by p_name) as avg,
-      |stddev(p_size) over(distribute by p_mfgr sort by p_name) as st,
-      |first_value(p_size % 5) over(distribute by p_mfgr sort by p_name) as fv,
-      |last_value(p_size) over(distribute by p_mfgr sort by p_name) as lv,
-      |first_value(p_size) over w1  as fvW1
-      |from part
-      |window w1 as (distribute by p_mfgr sort by p_mfgr, p_name
-      |             rows between 2 preceding and 2 following)
+    withSQLConf(SQLConf.LEGACY_CENTRAL_MOMENT_AGG.key -> "true") {
+      // Moved because:
+      // - Spark uses a different default stddev (sample instead of pop)
+      // - Tiny numerical differences in stddev results.
+      // - Different StdDev behavior when n=1 (NaN instead of 0)

Review comment:
       done

##########
File path: 
sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/expressions/aggregate/CentralMomentAgg.scala
##########
@@ -174,7 +175,9 @@ case class StddevSamp(child: Expression) extends 
CentralMomentAgg(child) {
 
   override val evaluateExpression: Expression = {
     If(n === 0.0, Literal.create(null, DoubleType),
-      If(n === 1.0, Double.NaN, sqrt(m2 / (n - 1.0))))
+      If(n === 1.0,
+        if (SQLConf.get.legacyCentralMomentAgg) Double.NaN else 
Literal.create(null, DoubleType),

Review comment:
       done




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