Github user HyukjinKwon commented on a diff in the pull request:
https://github.com/apache/spark/pull/15513#discussion_r85628075
--- Diff:
sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/expressions/windowExpressions.scala
---
@@ -401,22 +408,29 @@ case class Lead(input: Expression, offset:
Expression, default: Expression)
}
/**
- * The Lag function returns the value of 'x' at the 'offset'th row before
the current row in
+ * The Lag function returns the value of `input` at the `offset`th row
before the current row in
* the window. Offsets start at 0, which is the current row. The offset
must be constant
- * integer value. The default offset is 1. When the value of 'x' is null
at the 'offset'th row,
- * null is returned. If there is no such offset row, the default
expression is evaluated.
+ * integer value. The default offset is 1. When the value of `input` is
null at the `offset`th row,
+ * null is returned. If there is no such offset row, the `default`
expression is evaluated.
*
- * @param input expression to evaluate 'offset' rows before the current
row.
+ * @param input expression to evaluate `offset` rows before the current
row.
* @param offset rows to jump back in the partition.
* @param default to use when the offset row does not exist.
*/
-@ExpressionDescription(usage =
- """_FUNC_(input, offset, default) - LAG returns the value of 'x' at the
'offset'th row
- before the current row in the window.
- The default value of 'offset' is 1 and the default value of 'default'
is null.
- If the value of 'x' at the 'offset'th row is null, null is returned.
- If there is no such offset row (e.g. when the offset is 1, the first
row of the window
- does not have any previous row), 'default' is returned.""")
+@ExpressionDescription(
+ usage = """
+ _FUNC_(input[, offset[, default]]) - Returns the value of `input` at
the `offset`th row
+ before the current row in the window. The default value of `offset`
is 1 and the default
+ value of `default` is null. If the value of `input` at the
`offset`th row is null,
+ null is returned. If there is no such offset row (e.g., when the
offset is 1, the first
+ row of the window does not have any previous row), `default` is
returned.
+ """,
+ extended = """
+ Arguments:
+ input - an expression of any type.
+ offset - a numeric expression. Default is 1.
+ default - an expression of any type. Default is null.
--- End diff --
```scala
{
val df = Seq((1, "1"), (2, "2"), (1, "1"), (2, "2")).toDF("key",
"value")
.selectExpr("array(value) as value", "key")
df.select(
lag("value",
1).over(Window.partitionBy($"key").orderBy($"value"))).show()
}
{
val df = Seq((1, "1"), (2, "2"), (1, "1"), (2, "2")).toDF("key",
"value")
.selectExpr("struct(value) as value", "key")
df.select(
lag("value",
1).over(Window.partitionBy($"key").orderBy($"value"))).show()
}
```
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