[GitHub] [arrow-datafusion] alamb commented on a diff in pull request #6746: RFC: Make it easier to call window functions via expression API (and add example)

["alamb (via GitHub)" <gi...@apache.org>]

alamb commented on code in PR #6746:
URL: https://github.com/apache/arrow-datafusion/pull/6746#discussion_r1238815288


##########
datafusion/core/src/dataframe.rs:
##########
@@ -219,6 +219,23 @@ impl DataFrame {
     }
 
     /// Apply one or more window functions ([`Expr::WindowFunction`]) to extend the schema
+    ///
+    /// ```
+    /// # use datafusion::prelude::*;
+    /// # use datafusion::error::Result;
+    /// # #[tokio::main]
+    /// # async fn main() -> Result<()> {
+    /// let ctx = SessionContext::new();
+    /// let df = ctx.read_csv("tests/data/example.csv", CsvReadOptions::new()).await?;
+    ///
+    /// // The following is the equivalent of "SELECT FIRST_VALUE(b) OVER(PARTITION BY a)"
+    /// let first_value = first_value(col("b"))

Review Comment:
   The rest of the PR is motivated by trying to write this example. It turned out to be quite a pain to create the right `Expr`.



##########
datafusion/expr/src/expr.rs:
##########
@@ -459,22 +482,40 @@ pub struct WindowFunction {
 }
 
 impl WindowFunction {
-    /// Create a new Window expression
-    pub fn new(
-        fun: window_function::WindowFunction,
-        args: Vec<Expr>,
-        partition_by: Vec<Expr>,
-        order_by: Vec<Expr>,
-        window_frame: window_frame::WindowFrame,
-    ) -> Self {
+    /// Create a new Window expression with the specified argument an
+    /// empty `OVER` clause
+    pub fn new(fun: impl Into<window_function::WindowFunction>, args: Vec<Expr>) -> Self {

Review Comment:
   This is the major change -- so that WindowFunction acts like a `Builder` which can be used to set `partition by`, `order by` and `window_frame` with a more self documenting style



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