skambha commented on a change in pull request #27627: [WIP][SPARK-28067][SQL] 
Fix incorrect results for decimal aggregate sum by returning null on decimal 
overflow
URL: https://github.com/apache/spark/pull/27627#discussion_r380941152
 
 

 ##########
 File path: 
sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/expressions/aggregate/Sum.scala
 ##########
 @@ -60,38 +60,104 @@ case class Sum(child: Expression) extends 
DeclarativeAggregate with ImplicitCast
   private lazy val sumDataType = resultType
 
   private lazy val sum = AttributeReference("sum", sumDataType)()
+  private lazy val overflow = AttributeReference("overflow", BooleanType, 
false)()
 
   private lazy val zero = Literal.default(resultType)
 
-  override lazy val aggBufferAttributes = sum :: Nil
+  override lazy val aggBufferAttributes = sum :: overflow :: Nil
 
   override lazy val initialValues: Seq[Expression] = Seq(
-    /* sum = */ Literal.create(null, sumDataType)
+    /* sum = */ Literal.create(null, sumDataType),
+    /* overflow = */ Literal.create(false, BooleanType)
 
 Review comment:
   We keep track of overflow using this  aggBufferAttributes - overflow  to 
know if any of the intermediate add operations in updateExpressions and/or 
mergeExpressions overflow'd.  If the overflow is true and if 
spark.sql.ansi.enabled flag is false, then we return null for the sum operation 
in evaluateExpression.

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