Github user yijieshen commented on a diff in the pull request:
https://github.com/apache/spark/pull/6938#discussion_r34424137
--- Diff:
sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/expressions/math.scala
---
@@ -520,3 +522,202 @@ case class Logarithm(left: Expression, right:
Expression)
"""
}
}
+
+/**
+ * Round the `child`'s result to `scale` decimal place when `scale` >= 0
+ * or round at integral part when `scale` < 0.
+ * For example, round(31.415, 2) would eval to 31.42 and round(31.415, -1)
would eval to 30.
+ *
+ * Child of IntegralType would eval to itself when `scale` >= 0.
+ * Child of FractionalType whose value is NaN or Infinite would always
eval to itself.
+ *
+ * Round's dataType would always equal to `child`'s dataType except for
[[DecimalType.Fixed]],
+ * which leads to scale update in DecimalType's [[PrecisionInfo]]
+ *
+ * @param child expr to be round, all [[NumericType]] is allowed as Input
+ * @param scale new scale to be round to, this should be a constant int at
runtime
+ */
+case class Round(child: Expression, scale: Expression)
+ extends BinaryExpression with ExpectsInputTypes {
+
+ import BigDecimal.RoundingMode.HALF_UP
+
+ def this(child: Expression) = this(child, Literal(0))
+
+ override def left: Expression = child
+ override def right: Expression = scale
+
+ override def children: Seq[Expression] = Seq(child, scale)
+
+ // round of Decimal would eval to null if it fails to `changePrecision`
+ override def nullable: Boolean = true
+
+ override def foldable: Boolean = child.foldable
+
+ override lazy val dataType: DataType = child.dataType match {
+ case DecimalType.Fixed(p, s) => DecimalType(p, _scale)
--- End diff --
I've also noticed when the new scale is bigger than the original one,
decimal would just multiple the value by `10 ^ (new - origin)` but left `scale`
unchanged, may need to change the implementation in this situation.
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