Github user cloud-fan commented on a diff in the pull request:

    https://github.com/apache/spark/pull/18814#discussion_r131931907
  
    --- Diff: 
sql/core/src/main/scala/org/apache/spark/sql/expressions/WindowSpec.scala ---
    @@ -210,6 +210,59 @@ class WindowSpec private[sql](
       }
     
       /**
    +   * Defines the frame boundaries, 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 --
    
    ditto


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