Minor: Add additional docstrings to Window function implementations

datafusion/core/src/lib.rs

@@ -40,7 +40,7 @@
 //! # Examples
 //!
 //! The main entry point for interacting with DataFusion is the
-//! [`SessionContext`].
+//! [`SessionContext`]. [`Expr`]s represent expressions such as `a + b`.
 //!
 //! [`SessionContext`]: execution::context::SessionContext
 //!

datafusion/core/src/physical_plan/sorts/sort.rs

@@ -150,7 +150,7 @@ impl ExternalSorter {
             //
             // The factor of 2 aims to avoid a degenerate case where the
             // memory required for `fetch` is just under the memory available,
-            // causing repeated resorting of data
+            // causing repeated re-sorting of data
             if self.reservation.size() > before / 2
                 || self.reservation.try_grow(size).is_err()
             {

datafusion/expr/src/window_function.rs

@@ -73,7 +73,9 @@ impl fmt::Display for WindowFunction {
     }
 }
 
-/// An aggregate function that is part of a built-in window function
+/// A [window function] built in to DataFusion
+///
+/// [window function]: https://en.wikipedia.org/wiki/Window_function_(SQL)
 #[derive(Debug, Clone, PartialEq, Eq, Hash, EnumIter)]
 pub enum BuiltInWindowFunction {
     /// number of the current row within its partition, counting from 1

datafusion/physical-expr/src/window/aggregate.rs

@@ -37,12 +37,9 @@ use crate::{
     expressions::PhysicalSortExpr, reverse_order_bys, AggregateExpr, PhysicalExpr,
 };
 
-/// A window expr that takes the form of an aggregate function
-/// Aggregate Window Expressions that have the form
-/// `OVER({ROWS | RANGE| GROUPS} BETWEEN UNBOUNDED PRECEDING AND ...)`
-/// e.g cumulative window frames uses `PlainAggregateWindowExpr`. Where as Aggregate Window Expressions
-/// that have the form `OVER({ROWS | RANGE| GROUPS} BETWEEN M {PRECEDING| FOLLOWING} AND ...)`
-/// e.g sliding window frames uses `SlidingAggregateWindowExpr`.
+/// A window expr that takes the form of an aggregate function.
+///
+/// See comments on [`WindowExpr`] for more details.
 #[derive(Debug)]
 pub struct PlainAggregateWindowExpr {
     aggregate: Arc<dyn AggregateExpr>,

datafusion/physical-expr/src/window/built_in.rs

@@ -39,7 +39,7 @@ use datafusion_common::utils::evaluate_partition_ranges;
 use datafusion_common::{Result, ScalarValue};
 use datafusion_expr::WindowFrame;
 
-/// A window expr that takes the form of a built in window function
+/// A window expr that takes the form of a [`BuiltInWindowFunctionExpr`].
 #[derive(Debug)]
 pub struct BuiltInWindowExpr {
     expr: Arc<dyn BuiltInWindowFunctionExpr>,

datafusion/physical-expr/src/window/built_in_window_function_expr.rs

@@ -24,20 +24,24 @@ use datafusion_common::Result;
 use std::any::Any;
 use std::sync::Arc;
 
-/// A window expression that is a built-in window function.
+/// Evaluates a window function by instantiating a
+/// `[PartitionEvaluator]` for calculating the function's output in
+/// that partition.
 ///
-/// Note that unlike aggregation based window functions, built-in window functions normally ignore
-/// window frame spec, with the exception of first_value, last_value, and nth_value.
+/// Note that unlike aggregation based window functions, some window
+/// functions such as `rank` ignore the values in the window frame,
+/// but others such as `first_value`, `last_value`, and
+/// `nth_value` need the value.
+#[allow(rustdoc::private_intra_doc_links)]
 pub trait BuiltInWindowFunctionExpr: Send + Sync + std::fmt::Debug {
     /// Returns the aggregate expression as [`Any`](std::any::Any) so that it can be
     /// downcast to a specific implementation.
     fn as_any(&self) -> &dyn Any;
 
-    /// the field of the final result of this aggregation.
+    /// The field of the final result of evaluating this window function.
     fn field(&self) -> Result<Field>;
 
-    /// expressions that are passed to the Accumulator.
-    /// Single-column aggregations such as `sum` return a single value, others (e.g. `cov`) return many.
+    /// Expressions that are passed to the [`PartitionEvaluator`].
     fn expressions(&self) -> Vec<Arc<dyn PhysicalExpr>>;
 
     /// Human readable name such as `"MIN(c2)"` or `"RANK()"`. The default
@@ -46,8 +50,10 @@ pub trait BuiltInWindowFunctionExpr: Send + Sync + std::fmt::Debug {
         "BuiltInWindowFunctionExpr: default name"
     }
 
-    /// Evaluate window function arguments against the batch and return
-    /// an array ref. Typically, the resulting vector is a single element vector.
+    /// Evaluate window function's arguments against the input window
+    /// batch and return an [`ArrayRef`].
+    ///
+    /// Typically, the resulting vector is a single element vector.
     fn evaluate_args(&self, batch: &RecordBatch) -> Result<Vec<ArrayRef>> {
         self.expressions()
             .iter()
@@ -56,19 +62,36 @@ pub trait BuiltInWindowFunctionExpr: Send + Sync + std::fmt::Debug {
             .collect()
     }
 
-    /// Create built-in window evaluator with a batch
+    /// Create a [`PartitionEvaluator`] for evaluating the function on
+    /// a particular partition.
     fn create_evaluator(&self) -> Result<Box<dyn PartitionEvaluator>>;
 
-    /// Construct Reverse Expression that produces the same result
-    /// on a reversed window.  For example `lead(10)` --> `lag(10)`
+    /// Construct a new [`BuiltInWindowFunctionExpr`] that produces
+    /// the same result as this function on a window with reverse
+    /// order. The return value of this function is used by the
+    /// DataFusion optimizer to avoid re-sorting the data when
+    /// possible.
+    ///
+    /// Returns `None` (the default) if no reverse is known (or possible).
+    ///
+    /// For example, the reverse of `lead(10)` is `lag(10)`.
     fn reverse_expr(&self) -> Option<Arc<dyn BuiltInWindowFunctionExpr>> {
         None
     }
 
+    /// Can the window function be incrementally computed using
+    /// bounded memory?
+    ///
+    /// If this function returns true, [`Self::create_evaluator`] must
+    /// implement [`PartitionEvaluator::evaluate_stateful`]
     fn supports_bounded_execution(&self) -> bool {
         false
     }
 
+    /// Does the window function use the values from its window frame?
+    ///
+    /// If this function returns true, [`Self::create_evaluator`] must
+    /// implement [`PartitionEvaluator::evaluate_inside_range`]
     fn uses_window_frame(&self) -> bool {
         false
     }

datafusion/physical-expr/src/window/partition_evaluator.rs

@@ -15,7 +15,7 @@
 // specific language governing permissions and limitations
 // under the License.
 
-//! partition evaluation module
+//! Partition evaluation module
 
 use crate::window::window_expr::BuiltinWindowState;
 use crate::window::WindowAggState;
@@ -25,24 +25,97 @@ use datafusion_common::{DataFusionError, ScalarValue};
 use std::fmt::Debug;
 use std::ops::Range;
 
-/// Partition evaluator
+/// Partition evaluator for Window Functions
+///
+/// # Background
+///
+/// An implementation of this trait is created and used for each
+/// partition defined by an `OVER` clause and is instantiated by
+/// [`BuiltInWindowFunctionExpr::create_evaluator`]
+///
+/// For example, evaluating `window_func(val) OVER (PARTITION BY col)`
+/// on the following data:
+///
+/// ```text
+/// col | val
+/// --- + ----
+///  A  | 10
+///  A  | 10
+///  C  | 20
+///  D  | 30
+///  D  | 30
+/// ```
+///
+/// Will instantiate three `PartitionEvaluator`s, one each for the
+/// partitions defined by `col=A`, `col=B`, and `col=C`.
+///
+/// ```text
+/// col | val
+/// --- + ----
+///  A  | 10     <--- partition 1
+///  A  | 10
+///
+/// col | val
+/// --- + ----
+///  C  | 20     <--- partition 2
+///
+/// col | val
+/// --- + ----
+///  D  | 30     <--- partition 3
+///  D  | 30
+/// ```
+///
+/// Different methods on this trait will be called depending on the
+/// capabilities described by [`BuiltInWindowFunctionExpr`]:
+///
+/// # Stateless `PartitionEvaluator`
+///
+/// In this case, [`Self::evaluate`], [`Self::evaluate_with_rank`] or
+/// [`Self::evaluate_inside_range`] is called with values for the
+/// entire partition.
+///
+/// # Stateful `PartitionEvaluator`
+///
+/// In this case, [`Self::evaluate_stateful`] is called to calculate
+/// the results of the window function incrementally for each new
+/// batch, saving and restoring any state needed to do so as
+/// [`BuiltinWindowState`].
+///
+/// For example, when computing `ROW_NUMBER` incrementally,
+/// [`Self::evaluate_stateful`] will be called multiple times with
+/// different batches. For all batches after the first, the output
+/// `row_number` must start from last `row_number` produced for the
+/// previous batch. The previous row number is saved and restored as
+/// the state.
+///
+/// [`BuiltInWindowFunctionExpr`]: crate::window::BuiltInWindowFunctionExpr
+/// [`BuiltInWindowFunctionExpr::create_evaluator`]: crate::window::BuiltInWindowFunctionExpr::create_evaluator
 pub trait PartitionEvaluator: Debug + Send {
-    /// Whether the evaluator should be evaluated with rank
+    /// Can this evaluator be evaluated with (only) rank
+    ///
+    /// If `include_rank` is true, then [`Self::evaluate_with_rank`]
+    /// will be called for each partition, which includes the
+    /// `rank`.
     fn include_rank(&self) -> bool {
         false
     }
 
-    /// Returns state of the Built-in Window Function
+    /// Returns the internal state of the window function
+    ///
+    /// Only used for stateful evaluation
     fn state(&self) -> Result<BuiltinWindowState> {
         // If we do not use state we just return Default
         Ok(BuiltinWindowState::Default)
     }
 
-    /// Updates the internal state for Built-in window function
-    // state is useful to update internal state for Built-in window function.
-    // idx is the index of last row for which result is calculated.
-    // range_columns is the result of order by column values. It is used to calculate rank boundaries
-    // sort_partition_points is the boundaries of each rank in the range_column. It is used to update rank.
+    /// Updates the internal state for window function
+    ///
+    /// Only used for stateful evaluation
+    ///
+    /// `state`: is useful to update internal state for window function.
+    /// `idx`: is the index of last row for which result is calculated.
+    /// `range_columns`: is the result of order by column values. It is used to calculate rank boundaries
+    /// `sort_partition_points`: is the boundaries of each rank in the range_column. It is used to update rank.
     fn update_state(
         &mut self,
         _state: &WindowAggState,
@@ -54,36 +127,72 @@ pub trait PartitionEvaluator: Debug + Send {
         Ok(())
     }
 
+    /// Sets the internal state for window function
+    ///
+    /// Only used for stateful evaluation
     fn set_state(&mut self, _state: &BuiltinWindowState) -> Result<()> {
         Err(DataFusionError::NotImplemented(
             "set_state is not implemented for this window function".to_string(),
         ))
     }
 
-    /// Gets the range where Built-in window function result is calculated.
-    // idx is the index of last row for which result is calculated.
-    // n_rows is the number of rows of the input record batch (Used during bound check)
+    /// Gets the range where the window function result is calculated.
+    ///
+    /// `idx`: is the index of last row for which result is calculated.
+    /// `n_rows`: is the number of rows of the input record batch (Used during bounds check)
     fn get_range(&self, _idx: usize, _n_rows: usize) -> Result<Range<usize>> {
         Err(DataFusionError::NotImplemented(
             "get_range is not implemented for this window function".to_string(),
         ))
     }
 
-    /// Evaluate the partition evaluator against the partition
+    /// Called for window functions that *do not use* values from the
+    /// the window frame, such as `ROW_NUMBER`, `RANK`, `DENSE_RANK`,
+    /// `PERCENT_RANK`, `CUME_DIST`, `LEAD`, `LAG`).
     fn evaluate(&self, _values: &[ArrayRef], _num_rows: usize) -> Result<ArrayRef> {
         Err(DataFusionError::NotImplemented(
             "evaluate is not implemented by default".into(),
         ))
     }
 
-    /// Evaluate window function result inside given range
+    /// Evaluate window function result inside given range.
+    ///
+    /// Only used for stateful evaluation
     fn evaluate_stateful(&mut self, _values: &[ArrayRef]) -> Result<ScalarValue> {
         Err(DataFusionError::NotImplemented(
             "evaluate_stateful is not implemented by default".into(),
         ))
     }
 
-    /// evaluate the partition evaluator against the partition but with rank
+    /// [`PartitionEvaluator::evaluate_with_rank`] is called for window
+    /// functions that only need the rank of a row within its window
+    /// frame.
+    ///
+    /// Evaluate the partition evaluator against the partition using
+    /// the row ranks. For example, `RANK(col)` produces
+    ///
+    /// ```text
+    /// col | rank
+    /// --- + ----
+    ///  A  | 1
+    ///  A  | 1
+    ///  C  | 3
+    ///  D  | 4
+    ///  D  | 5
+    /// ```
+    ///
+    /// For this case, `num_rows` would be `5` and the
+    /// `ranks_in_partition` would be called with
+    ///
+    /// ```text
+    /// [
+    ///   (0,1),
+    ///   (2,2),
+    ///   (3,4),
+    /// ]
+    /// ```
+    ///
+    /// See [`Self::include_rank`] for more details
     fn evaluate_with_rank(
         &self,
         _num_rows: usize,
@@ -94,7 +203,11 @@ pub trait PartitionEvaluator: Debug + Send {
         ))
     }
 
-    /// evaluate window function result inside given range
+    /// Called for window functions that use values from window frame,
+    /// such as `FIRST_VALUE`, `LAST_VALUE`, `NTH_VALUE` and produce a
+    /// single value for every row in the partition.
+    ///
+    /// Returns a [`ScalarValue`] that is the value of the window function for the entire partition
     fn evaluate_inside_range(
         &self,
         _values: &[ArrayRef],

datafusion/physical-expr/src/window/sliding_aggregate.rs

@@ -36,12 +36,10 @@ use crate::{
     expressions::PhysicalSortExpr, reverse_order_bys, AggregateExpr, PhysicalExpr,
 };
 
-/// A window expr that takes the form of an aggregate function
-/// Aggregate Window Expressions that have the form
-/// `OVER({ROWS | RANGE| GROUPS} BETWEEN UNBOUNDED PRECEDING AND ...)`
-/// e.g cumulative window frames uses `PlainAggregateWindowExpr`. Where as Aggregate Window Expressions
-/// that have the form `OVER({ROWS | RANGE| GROUPS} BETWEEN M {PRECEDING| FOLLOWING} AND ...)`
-/// e.g sliding window frames uses `SlidingAggregateWindowExpr`.
+/// A window expr that takes the form of an aggregate function that
+/// can be incrementally computed over sliding windows.
+///
+/// See comments on [`WindowExpr`] for more details.
 #[derive(Debug)]
 pub struct SlidingAggregateWindowExpr {
     aggregate: Arc<dyn AggregateExpr>,
@@ -72,10 +70,11 @@ impl SlidingAggregateWindowExpr {
     }
 }
 
-/// peer based evaluation based on the fact that batch is pre-sorted given the sort columns
-/// and then per partition point we'll evaluate the peer group (e.g. SUM or MAX gives the same
-/// results for peers) and concatenate the results.
-
+/// Incrementally update window function using the fact that batch is
+/// pre-sorted given the sort columns and then per partition point.
+///
+/// Evaluates the peer group (e.g. `SUM` or `MAX` gives the same results
+/// for peers) and concatenate the results.
 impl WindowExpr for SlidingAggregateWindowExpr {
     /// Return a reference to Any that can be used for downcasting
     fn as_any(&self) -> &dyn Any {