[GitHub] [spark] Hisoka-X commented on a diff in pull request #41156: [SPARK-40129][SQL] Fix Decimal multiply can produce the wrong answer

["Hisoka-X (via GitHub)" <gi...@apache.org>]

Hisoka-X commented on code in PR #41156:
URL: https://github.com/apache/spark/pull/41156#discussion_r1280763963


##########
sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/expressions/arithmetic.scala:
##########
@@ -592,7 +607,10 @@ case class Multiply(
   override def inputType: AbstractDataType = NumericType
 
   override def symbol: String = "*"
-  override def decimalMethod: String = "$times"
+
+  override def decimalMethod(mathContextValue: GlobalValue, value1: String, value2: String):
+    String = s"Decimal.apply($value1.toJavaBigDecimal()" +
+      s".multiply($value2.toJavaBigDecimal(), $mathContextValue))"

Review Comment:
   > Is it better to avoid double round-up?
   
   @cloud-fan In fact, the current solution is very similar to what you mentioned, we pass in a wider `MathContext`, and then do RoundUp in `toPrecision`.
   



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