NTH_VALUE reverse support

datafusion/core/src/physical_optimizer/enforce_distribution.rs

@@ -28,8 +28,8 @@ use std::sync::Arc;
 use crate::config::ConfigOptions;
 use crate::error::Result;
 use crate::physical_optimizer::utils::{
-    add_sort_above, get_children_exectrees, get_plan_string, is_coalesce_partitions,
-    is_repartition, is_sort_preserving_merge, ExecTree,
+    add_sort_above, get_children_exectrees, is_coalesce_partitions, is_repartition,
+    is_sort_preserving_merge, ExecTree,
 };
 use crate::physical_optimizer::PhysicalOptimizerRule;
 use crate::physical_plan::aggregates::{AggregateExec, AggregateMode, PhysicalGroupBy};
@@ -54,8 +54,8 @@ use datafusion_physical_expr::utils::map_columns_before_projection;
 use datafusion_physical_expr::{
     physical_exprs_equal, EquivalenceProperties, PhysicalExpr,
 };
-use datafusion_physical_plan::unbounded_output;
 use datafusion_physical_plan::windows::{get_best_fitting_window, BoundedWindowAggExec};
+use datafusion_physical_plan::{get_plan_string, unbounded_output};
 
 use itertools::izip;
 

datafusion/core/src/physical_optimizer/enforce_sorting.rs

@@ -763,9 +763,8 @@ mod tests {
         repartition_exec, sort_exec, sort_expr, sort_expr_options, sort_merge_join_exec,
         sort_preserving_merge_exec, spr_repartition_exec, union_exec,
     };
-    use crate::physical_optimizer::utils::get_plan_string;
     use crate::physical_plan::repartition::RepartitionExec;
-    use crate::physical_plan::{displayable, Partitioning};
+    use crate::physical_plan::{displayable, get_plan_string, Partitioning};
     use crate::prelude::{SessionConfig, SessionContext};
     use crate::test::csv_exec_sorted;
 

datafusion/core/src/physical_optimizer/projection_pushdown.rs

@@ -1089,7 +1089,6 @@ mod tests {
     use crate::physical_optimizer::projection_pushdown::{
         join_table_borders, update_expr, ProjectionPushdown,
     };
-    use crate::physical_optimizer::utils::get_plan_string;
     use crate::physical_optimizer::PhysicalOptimizerRule;
     use crate::physical_plan::coalesce_partitions::CoalescePartitionsExec;
     use crate::physical_plan::filter::FilterExec;
@@ -1100,7 +1099,7 @@ mod tests {
     use crate::physical_plan::repartition::RepartitionExec;
     use crate::physical_plan::sorts::sort::SortExec;
     use crate::physical_plan::sorts::sort_preserving_merge::SortPreservingMergeExec;
-    use crate::physical_plan::ExecutionPlan;
+    use crate::physical_plan::{get_plan_string, ExecutionPlan};
 
     use arrow_schema::{DataType, Field, Schema, SortOptions};
     use datafusion_common::config::ConfigOptions;

datafusion/core/src/physical_optimizer/replace_with_order_preserving_variants.rs

@@ -286,7 +286,7 @@ mod tests {
     use crate::physical_plan::repartition::RepartitionExec;
     use crate::physical_plan::sorts::sort::SortExec;
     use crate::physical_plan::sorts::sort_preserving_merge::SortPreservingMergeExec;
-    use crate::physical_plan::{displayable, Partitioning};
+    use crate::physical_plan::{displayable, get_plan_string, Partitioning};
     use crate::prelude::SessionConfig;
 
     use arrow::compute::SortOptions;
@@ -929,11 +929,4 @@ mod tests {
             FileCompressionType::UNCOMPRESSED,
         ))
     }
-
-    // Util function to get string representation of a physical plan
-    fn get_plan_string(plan: &Arc<dyn ExecutionPlan>) -> Vec<String> {
-        let formatted = displayable(plan.as_ref()).indent(true).to_string();
-        let actual: Vec<&str> = formatted.trim().lines().collect();
-        actual.iter().map(|elem| elem.to_string()).collect()
-    }
 }

datafusion/core/src/physical_optimizer/utils.rs

@@ -28,7 +28,7 @@ use crate::physical_plan::sorts::sort::SortExec;
 use crate::physical_plan::sorts::sort_preserving_merge::SortPreservingMergeExec;
 use crate::physical_plan::union::UnionExec;
 use crate::physical_plan::windows::{BoundedWindowAggExec, WindowAggExec};
-use crate::physical_plan::{displayable, ExecutionPlan};
+use crate::physical_plan::{get_plan_string, ExecutionPlan};
 
 use datafusion_physical_expr::{LexRequirementRef, PhysicalSortRequirement};
 
@@ -154,10 +154,3 @@ pub fn is_union(plan: &Arc<dyn ExecutionPlan>) -> bool {
 pub fn is_repartition(plan: &Arc<dyn ExecutionPlan>) -> bool {
     plan.as_any().is::<RepartitionExec>()
 }
-
-/// Utility function yielding a string representation of the given [`ExecutionPlan`].
-pub fn get_plan_string(plan: &Arc<dyn ExecutionPlan>) -> Vec<String> {
-    let formatted = displayable(plan.as_ref()).indent(true).to_string();
-    let actual: Vec<&str> = formatted.trim().lines().collect();
-    actual.iter().map(|elem| elem.to_string()).collect()
-}

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

@@ -15,21 +15,24 @@
 // specific language governing permissions and limitations
 // under the License.
 
-//! Defines physical expressions for `first_value`, `last_value`, and `nth_value`
-//! that can evaluated at runtime during query execution
+//! Defines physical expressions for `FIRST_VALUE`, `LAST_VALUE`, and `NTH_VALUE`
+//! functions that can be evaluated at run time during query execution.
+
+use std::any::Any;
+use std::cmp::Ordering;
+use std::ops::Range;
+use std::sync::Arc;
 
 use crate::window::window_expr::{NthValueKind, NthValueState};
 use crate::window::BuiltInWindowFunctionExpr;
 use crate::PhysicalExpr;
+
 use arrow::array::{Array, ArrayRef};
 use arrow::datatypes::{DataType, Field};
 use datafusion_common::{exec_err, ScalarValue};
 use datafusion_common::{DataFusionError, Result};
 use datafusion_expr::window_state::WindowAggState;
 use datafusion_expr::PartitionEvaluator;
-use std::any::Any;
-use std::ops::Range;
-use std::sync::Arc;
 
 /// nth_value expression
 #[derive(Debug)]
@@ -77,17 +80,17 @@ impl NthValue {
         n: u32,
     ) -> Result<Self> {
         match n {
-            0 => exec_err!("nth_value expect n to be > 0"),
+            0 => exec_err!("NTH_VALUE expects n to be non-zero"),
             _ => Ok(Self {
                 name: name.into(),
                 expr,
                 data_type,
-                kind: NthValueKind::Nth(n),
+                kind: NthValueKind::Nth(n as i64),
             }),
         }
     }
 
-    /// Get nth_value kind
+    /// Get the NTH_VALUE kind
     pub fn get_kind(&self) -> NthValueKind {
         self.kind
     }
@@ -125,7 +128,7 @@ impl BuiltInWindowFunctionExpr for NthValue {
         let reversed_kind = match self.kind {
             NthValueKind::First => NthValueKind::Last,
             NthValueKind::Last => NthValueKind::First,
-            NthValueKind::Nth(_) => return None,
+            NthValueKind::Nth(idx) => NthValueKind::Nth(-idx),
         };
         Some(Arc::new(Self {
             name: self.name.clone(),
@@ -143,16 +146,17 @@ pub(crate) struct NthValueEvaluator {
 }
 
 impl PartitionEvaluator for NthValueEvaluator {
-    /// When the window frame has a fixed beginning (e.g UNBOUNDED
-    /// PRECEDING), for some functions such as FIRST_VALUE, LAST_VALUE and
-    /// NTH_VALUE we can memoize result.  Once result is calculated it
-    /// will always stay same. Hence, we do not need to keep past data
-    /// as we process the entire dataset. This feature enables us to
-    /// prune rows from table. The default implementation does nothing
+    /// When the window frame has a fixed beginning (e.g UNBOUNDED PRECEDING),
+    /// for some functions such as FIRST_VALUE, LAST_VALUE and NTH_VALUE, we
+    /// can memoize the result.  Once result is calculated, it will always stay
+    /// same. Hence, we do not need to keep past data as we process the entire
+    /// dataset.
     fn memoize(&mut self, state: &mut WindowAggState) -> Result<()> {
         let out = &state.out_col;
         let size = out.len();
-        let (is_prunable, is_last) = match self.state.kind {
+        let mut buffer_size = 1;
+        // Decide if we arrived at a final result yet:
+        let (is_prunable, is_reverse_direction) = match self.state.kind {
             NthValueKind::First => {
                 let n_range =
                     state.window_frame_range.end - state.window_frame_range.start;
@@ -162,16 +166,30 @@ impl PartitionEvaluator for NthValueEvaluator {
             NthValueKind::Nth(n) => {
                 let n_range =
                     state.window_frame_range.end - state.window_frame_range.start;
-                (n_range >= (n as usize) && size >= (n as usize), false)
+                match n.cmp(&0) {
+                    Ordering::Greater => {
+                        (n_range >= (n as usize) && size > (n as usize), false)
+                    }
+                    Ordering::Less => {
+                        let reverse_index = (-n) as usize;
+                        buffer_size = reverse_index;
+                        // Negative index represents reverse direction.
+                        (n_range >= reverse_index, true)
+                    }
+                    Ordering::Equal => {
+                        // The case n = 0 is not valid for the NTH_VALUE function.
+                        unreachable!();
+                    }
+                }
             }
         };
         if is_prunable {
-            if self.state.finalized_result.is_none() && !is_last {
+            if self.state.finalized_result.is_none() && !is_reverse_direction {
                 let result = ScalarValue::try_from_array(out, size - 1)?;
                 self.state.finalized_result = Some(result);
             }
             state.window_frame_range.start =
-                state.window_frame_range.end.saturating_sub(1);
+                state.window_frame_range.end.saturating_sub(buffer_size);
         }
         Ok(())
     }
@@ -195,12 +213,33 @@ impl PartitionEvaluator for NthValueEvaluator {
                 NthValueKind::First => ScalarValue::try_from_array(arr, range.start),
                 NthValueKind::Last => ScalarValue::try_from_array(arr, range.end - 1),
                 NthValueKind::Nth(n) => {
-                    // We are certain that n > 0.
-                    let index = (n as usize) - 1;
-                    if index >= n_range {
-                        ScalarValue::try_from(arr.data_type())
-                    } else {
-                        ScalarValue::try_from_array(arr, range.start + index)
+                    match n.cmp(&0) {
+                        Ordering::Greater => {
+                            // SQL indices are not 0-based.
+                            let index = (n as usize) - 1;
+                            if index >= n_range {
+                                // Outside the range, return NULL:
+                                ScalarValue::try_from(arr.data_type())
+                            } else {
+                                ScalarValue::try_from_array(arr, range.start + index)
+                            }
+                        }
+                        Ordering::Less => {
+                            let reverse_index = (-n) as usize;
+                            if n_range >= reverse_index {
+                                ScalarValue::try_from_array(
+                                    arr,
+                                    range.start + n_range - reverse_index,
+                                )
+                            } else {
+                                // Outside the range, return NULL:
+                                ScalarValue::try_from(arr.data_type())
+                            }
+                        }
+                        Ordering::Equal => {
+                            // The case n = 0 is not valid for the NTH_VALUE function.
+                            unreachable!();
+                        }
                     }
                 }
             }

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

@@ -15,7 +15,13 @@
 // specific language governing permissions and limitations
 // under the License.
 
+use std::any::Any;
+use std::fmt::Debug;
+use std::ops::Range;
+use std::sync::Arc;
+
 use crate::{PhysicalExpr, PhysicalSortExpr};
+
 use arrow::array::{new_empty_array, Array, ArrayRef};
 use arrow::compute::kernels::sort::SortColumn;
 use arrow::compute::SortOptions;
@@ -25,13 +31,9 @@ use datafusion_common::{internal_err, DataFusionError, Result, ScalarValue};
 use datafusion_expr::window_state::{
     PartitionBatchState, WindowAggState, WindowFrameContext,
 };
-use datafusion_expr::PartitionEvaluator;
-use datafusion_expr::{Accumulator, WindowFrame};
+use datafusion_expr::{Accumulator, PartitionEvaluator, WindowFrame};
+
 use indexmap::IndexMap;
-use std::any::Any;
-use std::fmt::Debug;
-use std::ops::Range;
-use std::sync::Arc;
 
 /// Common trait for [window function] implementations
 ///
@@ -292,7 +294,7 @@ pub struct NumRowsState {
 pub enum NthValueKind {
     First,
     Last,
-    Nth(u32),
+    Nth(i64),
 }
 
 #[derive(Debug, Clone)]

datafusion/physical-plan/src/lib.rs

@@ -570,5 +570,12 @@ pub fn unbounded_output(plan: &Arc<dyn ExecutionPlan>) -> bool {
         .unwrap_or(true)
 }
 
+/// Utility function yielding a string representation of the given [`ExecutionPlan`].
+pub fn get_plan_string(plan: &Arc<dyn ExecutionPlan>) -> Vec<String> {
+    let formatted = displayable(plan.as_ref()).indent(true).to_string();
+    let actual: Vec<&str> = formatted.trim().lines().collect();
+    actual.iter().map(|elem| elem.to_string()).collect()
+}
+
 #[cfg(test)]
 pub mod test;