Minor: Document output schema of LogicalPlan::Aggregate and LogicalPlan::Window (#14047)

Author: alamb

datafusion/expr/src/logical_plan/plan.rs

@@ -215,10 +215,14 @@ pub enum LogicalPlan {
     /// Windows input based on a set of window spec and window
     /// function (e.g. SUM or RANK).  This is used to implement SQL
     /// window functions, and the `OVER` clause.
+    ///
+    /// See [`Window`] for more details
     Window(Window),
     /// Aggregates its input based on a set of grouping and aggregate
     /// expressions (e.g. SUM). This is used to implement SQL aggregates
     /// and `GROUP BY`.
+    ///
+    /// See [`Aggregate`] for more details
     Aggregate(Aggregate),
     /// Sorts its input according to a list of sort expressions. This
     /// is used to implement SQL `ORDER BY`
@@ -2365,6 +2369,19 @@ impl Filter {
 }
 
 /// Window its input based on a set of window spec and window function (e.g. SUM or RANK)
+///
+/// # Output Schema
+///
+/// The output schema is the input schema followed by the window function
+/// expressions, in order.
+///
+/// For example, given the input schema `"A", "B", "C"` and the window function
+/// `SUM(A) OVER (PARTITION BY B+1 ORDER BY C)`, the output schema will be `"A",
+/// "B", "C", "SUM(A) OVER ..."` where `"SUM(A) OVER ..."` is the name of the
+/// output column.
+///
+/// Note that the `PARTITION BY` expression "B+1" is not produced in the output
+/// schema.
 #[derive(Debug, Clone, PartialEq, Eq, Hash)]
 pub struct Window {
     /// The incoming logical plan
@@ -2968,6 +2985,16 @@ impl PartialOrd for DistinctOn {
 
 /// Aggregates its input based on a set of grouping and aggregate
 /// expressions (e.g. SUM).
+///
+/// # Output Schema
+///
+/// The output schema is the group expressions followed by the aggregate
+/// expressions in order.
+///
+/// For example, given the input schema `"A", "B", "C"` and the aggregate
+/// `SUM(A) GROUP BY C+B`, the output schema will be `"C+B", "SUM(A)"` where
+/// "C+B" and "SUM(A)" are the names of the output columns. Note that "C+B" is a
+/// single new column
 #[derive(Debug, Clone, PartialEq, Eq, Hash)]
 // mark non_exhaustive to encourage use of try_new/new()
 #[non_exhaustive]