beliefer commented on a change in pull request #29604:
URL: https://github.com/apache/spark/pull/29604#discussion_r489123677
##########
File path:
sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/expressions/windowExpressions.scala
##########
@@ -549,6 +549,96 @@ case class CumeDist() extends RowNumberLike with
SizeBasedWindowFunction {
override def prettyName: String = "cume_dist"
}
+/**
+ * The NthValue function returns the value of `input` at the `offset`th row
from beginning of the
+ * window frame. Offset starts at 1. When the value of `input` is null at the
`offset`th row or
+ * there is no such an `offset`th row, null is returned.
+ */
+@ExpressionDescription(
+ usage = """
+ _FUNC_(input[, offset]) - Returns the value of `input` at the row that is
the `offset`th row
+ from beginning of the window frame. Offsets start at 1. If the value of
`input` at the
+ `offset`th row is null, null is returned. If there is no such an offset
row (e.g., when the
+ offset is 10, size of the window frame less than 10), null is returned.
+ """,
+ arguments = """
+ Arguments:
+ * input - the target column or expression that the function operates on.
+ * offset - an int expression which determines the row number relative to
the first row in
+ the window for which to return the expression. The offset can be a
constant or an
+ expression and must be a positive integer that is greater than 0.
+ * ignoreNulls - an optional specification that indicates the NthValue
should skip null
Review comment:
How about `If the value of `input` at the `offset`th row is null, null
is returned (respecting nulls)`
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