Github user cloud-fan commented on a diff in the pull request:
https://github.com/apache/spark/pull/18814#discussion_r131931683
--- Diff:
sql/core/src/main/scala/org/apache/spark/sql/expressions/Window.scala ---
@@ -210,6 +210,57 @@ object Window {
spec.rangeBetween(start, end)
}
+ /**
+ * Creates a [[WindowSpec]] with the frame boundaries defined,
+ * from `start` (inclusive) to `end` (inclusive).
+ *
+ * Both `start` and `end` are relative to the current row. For example,
"lit(0)" means
+ * "current row", while "lit(-1)" means one off before the current row,
and "lit(5)" means the
+ * five off after the current row.
+ *
+ * Users should use `unboundedPreceding()`, `unboundedFollowing()`, and
`currentRow()` from
+ * [[org.apache.spark.sql.functions]] to specify special boundary
values, literals are not
+ * transformed to
[[org.apache.spark.sql.catalyst.expressions.SpecialFrameBoundary]]s.
+ *
+ * A range-based boundary is based on the actual value of the ORDER BY
+ * expression(s). An offset is used to alter the value of the ORDER BY
expression, for
+ * instance if the current order by expression has a value of 10 and the
lower bound offset
+ * is -3, the resulting lower bound for the current row will be 10 - 3 =
7. This however puts a
+ * number of constraints on the ORDER BY expressions: there can be only
one expression and this
+ * expression must have a numerical/date/timestamp data type. An
exception can be made when the
+ * offset is unbounded, because no value modification is needed, in this
case multiple and
+ * non-literal ORDER BY expression are allowed.
--- End diff --
non-numerical/date/timestamp data type
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