feat(optimizer): Enable filter pushdown on window functions

datafusion/optimizer/Cargo.toml

@@ -43,6 +43,7 @@ arrow = { workspace = true }
 chrono = { workspace = true }
 datafusion-common = { workspace = true, default-features = true }
 datafusion-expr = { workspace = true }
+datafusion-functions-window = { workspace = true }
 datafusion-physical-expr = { workspace = true }
 indexmap = { workspace = true }
 itertools = { workspace = true }

datafusion/optimizer/src/push_down_filter.rs

@@ -985,6 +985,77 @@ impl OptimizerRule for PushDownFilter {
                         }
                     })
             }
+            LogicalPlan::Window(window) => {
+                // Retrieve the set of potential partition keys where we can push filters by.
+                // Unlike aggregations, where there is only one statement per SELECT, there can be
+                // multiple window functions, each with potentially different partition keys.
+                // Therefore, we need to ensure that any potential partition key returned is used in
+                // ALL window functions. Otherwise, filters cannot be pushed by through that column.
+                let potential_partition_keys = window
+                    .window_expr
+                    .iter()
+                    .map(|e| {
+                        if let Expr::WindowFunction(window_expression) = e {
+                            window_expression
+                                .partition_by
+                                .iter()
+                                .map(|c| {
+                                    Column::from_qualified_name(
+                                        c.schema_name().to_string(),
+                                    )
+                                })
+                                .collect::<HashSet<_>>()
+                        } else {
+                            // window functions expressions are only Expr::WindowFunction
+                            unreachable!()
+                        }
+                    })
+                    // performs the set intersection of the partition keys of all window functions,
+                    // returning only the common ones
+                    .reduce(|a, b| &a & &b)
+                    .unwrap_or_default();
+
+                let predicates = split_conjunction_owned(filter.predicate);
+                let mut keep_predicates = vec![];
+                let mut push_predicates = vec![];
+                for expr in predicates {
+                    let cols = expr.column_refs();
+                    if cols.iter().all(|c| potential_partition_keys.contains(c)) {
+                        push_predicates.push(expr);
+                    } else {
+                        keep_predicates.push(expr);
+                    }
+                }
+
+                // Unlike with aggregations, there are no cases where we have to replace, e.g.,
+                // `a+b` with Column(a)+Column(b). This is because partition expressions are not
+                // available as standalone columns to the user. For example, while an aggregation on
+                // `a+b` becomes Column(a + b), in a window partition it becomes
+                // `func() PARTITION BY [a + b] ...`. Thus, filters on expressions always remain in
+                // place, so we can use `push_predicates` directly. This is consistent with other
+                // optimizers, such as the one used by Postgres.
+
+                let window_input = Arc::clone(&window.input);
+                Transformed::yes(LogicalPlan::Window(window))
+                    .transform_data(|new_plan| {
+                        // If we have a filter to push, we push it down to the input of the window
+                        if let Some(predicate) = conjunction(push_predicates) {
+                            let new_filter = make_filter(predicate, window_input)?;
+                            insert_below(new_plan, new_filter)
+                        } else {
+                            Ok(Transformed::no(new_plan))
+                        }
+                    })?
+                    .map_data(|child_plan| {
+                        // if there are any remaining predicates we can't push, add them
+                        // back as a filter
+                        if let Some(predicate) = conjunction(keep_predicates) {
+                            make_filter(predicate, Arc::new(child_plan))
+                        } else {
+                            Ok(child_plan)
+                        }
+                    })
+            }
             LogicalPlan::Join(join) => push_down_join(join, Some(&filter.predicate)),
             LogicalPlan::TableScan(scan) => {
                 let filter_predicates = split_conjunction(&filter.predicate);
@@ -1289,12 +1360,12 @@ mod tests {
     use async_trait::async_trait;
 
     use datafusion_common::{DFSchemaRef, ScalarValue};
-    use datafusion_expr::expr::ScalarFunction;
+    use datafusion_expr::expr::{ScalarFunction, WindowFunction};
     use datafusion_expr::logical_plan::table_scan;
     use datafusion_expr::{
-        col, in_list, in_subquery, lit, ColumnarValue, Extension, LogicalPlanBuilder,
-        ScalarUDF, ScalarUDFImpl, Signature, TableSource, TableType,
-        UserDefinedLogicalNodeCore, Volatility,
+        col, in_list, in_subquery, lit, ColumnarValue, ExprFunctionExt, Extension,
+        LogicalPlanBuilder, ScalarUDF, ScalarUDFImpl, Signature, TableSource, TableType,
+        UserDefinedLogicalNodeCore, Volatility, WindowFunctionDefinition,
     };
 
     use crate::optimizer::Optimizer;
@@ -1442,6 +1513,227 @@ mod tests {
         assert_optimized_plan_eq(plan, expected)
     }
 
+    /// verifies that when partitioning by 'a' and 'b', and filtering by 'b', 'b' is pushed
+    #[test]
+    fn filter_move_window() -> Result<()> {
+        let table_scan = test_table_scan()?;
+
+        let window = Expr::WindowFunction(WindowFunction::new(
+            WindowFunctionDefinition::WindowUDF(
+                datafusion_functions_window::rank::rank_udwf(),
+            ),
+            vec![],
+        ))
+        .partition_by(vec![col("a"), col("b")])
+        .order_by(vec![col("c").sort(true, true)])
+        .build()
+        .unwrap();
+
+        let plan = LogicalPlanBuilder::from(table_scan)
+            .window(vec![window])?
+            .filter(col("b").gt(lit(10i64)))?
+            .build()?;
+
+        let expected = "\
+        WindowAggr: windowExpr=[[rank() PARTITION BY [test.a, test.b] ORDER BY [test.c ASC NULLS FIRST] ROWS BETWEEN UNBOUNDED PRECEDING AND CURRENT ROW]]\
+        \n  TableScan: test, full_filters=[test.b > Int64(10)]";
+        assert_optimized_plan_eq(plan, expected)
+    }
+
+    /// verifies that when partitioning by 'a' and 'b', and filtering by 'a' and 'b', both 'a' and
+    /// 'b' are pushed
+    #[test]
+    fn filter_move_complex_window() -> Result<()> {
+        let table_scan = test_table_scan()?;
+
+        let window = Expr::WindowFunction(WindowFunction::new(
+            WindowFunctionDefinition::WindowUDF(
+                datafusion_functions_window::rank::rank_udwf(),
+            ),
+            vec![],
+        ))
+        .partition_by(vec![col("a"), col("b")])
+        .order_by(vec![col("c").sort(true, true)])
+        .build()
+        .unwrap();
+
+        let plan = LogicalPlanBuilder::from(table_scan)
+            .window(vec![window])?
+            .filter(and(col("a").gt(lit(10i64)), col("b").eq(lit(1i64))))?
+            .build()?;
+
+        let expected = "\
+            WindowAggr: windowExpr=[[rank() PARTITION BY [test.a, test.b] ORDER BY [test.c ASC NULLS FIRST] ROWS BETWEEN UNBOUNDED PRECEDING AND CURRENT ROW]]\
+            \n  TableScan: test, full_filters=[test.a > Int64(10), test.b = Int64(1)]";
+        assert_optimized_plan_eq(plan, expected)
+    }
+
+    /// verifies that when partitioning by 'a' and filtering by 'a' and 'b', only 'a' is pushed
+    #[test]
+    fn filter_move_partial_window() -> Result<()> {
+        let table_scan = test_table_scan()?;
+
+        let window = Expr::WindowFunction(WindowFunction::new(
+            WindowFunctionDefinition::WindowUDF(
+                datafusion_functions_window::rank::rank_udwf(),
+            ),
+            vec![],
+        ))
+        .partition_by(vec![col("a")])
+        .order_by(vec![col("c").sort(true, true)])
+        .build()
+        .unwrap();
+
+        let plan = LogicalPlanBuilder::from(table_scan)
+            .window(vec![window])?
+            .filter(and(col("a").gt(lit(10i64)), col("b").eq(lit(1i64))))?
+            .build()?;
+
+        let expected = "\
+        Filter: test.b = Int64(1)\
+        \n  WindowAggr: windowExpr=[[rank() PARTITION BY [test.a] ORDER BY [test.c ASC NULLS FIRST] ROWS BETWEEN UNBOUNDED PRECEDING AND CURRENT ROW]]\
+        \n    TableScan: test, full_filters=[test.a > Int64(10)]";
+        assert_optimized_plan_eq(plan, expected)
+    }
+
+    /// verifies that filters on partition expressions are not pushed, as the single expression
+    /// column is not available to the user, unlike with aggregations
+    #[test]
+    fn filter_expression_keep_window() -> Result<()> {
+        let table_scan = test_table_scan()?;
+
+        let window = Expr::WindowFunction(WindowFunction::new(
+            WindowFunctionDefinition::WindowUDF(
+                datafusion_functions_window::rank::rank_udwf(),
+            ),
+            vec![],
+        ))
+        .partition_by(vec![add(col("a"), col("b"))]) // PARTITION BY a + b
+        .order_by(vec![col("c").sort(true, true)])
+        .build()
+        .unwrap();
+
+        let plan = LogicalPlanBuilder::from(table_scan)
+            .window(vec![window])?
+            // unlike with aggregations, single partition column "test.a + test.b" is not available
+            // to the plan, so we use multiple columns when filtering
+            .filter(add(col("a"), col("b")).gt(lit(10i64)))?
+            .build()?;
+
+        let expected = "\
+        Filter: test.a + test.b > Int64(10)\
+        \n  WindowAggr: windowExpr=[[rank() PARTITION BY [test.a + test.b] ORDER BY [test.c ASC NULLS FIRST] ROWS BETWEEN UNBOUNDED PRECEDING AND CURRENT ROW]]\
+        \n    TableScan: test";
+        assert_optimized_plan_eq(plan, expected)
+    }
+
+    /// verifies that filters are not pushed on order by columns (that are not used in partitioning)
+    #[test]
+    fn filter_order_keep_window() -> Result<()> {
+        let table_scan = test_table_scan()?;
+
+        let window = Expr::WindowFunction(WindowFunction::new(
+            WindowFunctionDefinition::WindowUDF(
+                datafusion_functions_window::rank::rank_udwf(),
+            ),
+            vec![],
+        ))
+        .partition_by(vec![col("a")])
+        .order_by(vec![col("c").sort(true, true)])
+        .build()
+        .unwrap();
+
+        let plan = LogicalPlanBuilder::from(table_scan)
+            .window(vec![window])?
+            .filter(col("c").gt(lit(10i64)))?
+            .build()?;
+
+        let expected = "\
+        Filter: test.c > Int64(10)\
+        \n  WindowAggr: windowExpr=[[rank() PARTITION BY [test.a] ORDER BY [test.c ASC NULLS FIRST] ROWS BETWEEN UNBOUNDED PRECEDING AND CURRENT ROW]]\
+        \n    TableScan: test";
+        assert_optimized_plan_eq(plan, expected)
+    }
+
+    /// verifies that when we use multiple window functions with a common partition key, the filter
+    /// on that key is pushed
+    #[test]
+    fn filter_multiple_windows_common_partitions() -> Result<()> {
+        let table_scan = test_table_scan()?;
+
+        let window1 = Expr::WindowFunction(WindowFunction::new(
+            WindowFunctionDefinition::WindowUDF(
+                datafusion_functions_window::rank::rank_udwf(),
+            ),
+            vec![],
+        ))
+        .partition_by(vec![col("a")])
+        .order_by(vec![col("c").sort(true, true)])
+        .build()
+        .unwrap();
+
+        let window2 = Expr::WindowFunction(WindowFunction::new(
+            WindowFunctionDefinition::WindowUDF(
+                datafusion_functions_window::rank::rank_udwf(),
+            ),
+            vec![],
+        ))
+        .partition_by(vec![col("b"), col("a")])
+        .order_by(vec![col("c").sort(true, true)])
+        .build()
+        .unwrap();
+
+        let plan = LogicalPlanBuilder::from(table_scan)
+            .window(vec![window1, window2])?
+            .filter(col("a").gt(lit(10i64)))? // a appears in both window functions
+            .build()?;
+
+        let expected = "\
+            WindowAggr: windowExpr=[[rank() PARTITION BY [test.a] ORDER BY [test.c ASC NULLS FIRST] ROWS BETWEEN UNBOUNDED PRECEDING AND CURRENT ROW, rank() PARTITION BY [test.b, test.a] ORDER BY [test.c ASC NULLS FIRST] ROWS BETWEEN UNBOUNDED PRECEDING AND CURRENT ROW]]\
+            \n  TableScan: test, full_filters=[test.a > Int64(10)]";
+        assert_optimized_plan_eq(plan, expected)
+    }
+
+    /// verifies that when we use multiple window functions with different partitions keys, the
+    /// filter cannot be pushed
+    #[test]
+    fn filter_multiple_windows_disjoint_partitions() -> Result<()> {
+        let table_scan = test_table_scan()?;
+
+        let window1 = Expr::WindowFunction(WindowFunction::new(
+            WindowFunctionDefinition::WindowUDF(
+                datafusion_functions_window::rank::rank_udwf(),
+            ),
+            vec![],
+        ))
+        .partition_by(vec![col("a")])
+        .order_by(vec![col("c").sort(true, true)])
+        .build()
+        .unwrap();
+
+        let window2 = Expr::WindowFunction(WindowFunction::new(
+            WindowFunctionDefinition::WindowUDF(
+                datafusion_functions_window::rank::rank_udwf(),
+            ),
+            vec![],
+        ))
+        .partition_by(vec![col("b"), col("a")])
+        .order_by(vec![col("c").sort(true, true)])
+        .build()
+        .unwrap();
+
+        let plan = LogicalPlanBuilder::from(table_scan)
+            .window(vec![window1, window2])?
+            .filter(col("b").gt(lit(10i64)))? // b only appears in one window function
+            .build()?;
+
+        let expected = "\
+            Filter: test.b > Int64(10)\
+            \n  WindowAggr: windowExpr=[[rank() PARTITION BY [test.a] ORDER BY [test.c ASC NULLS FIRST] ROWS BETWEEN UNBOUNDED PRECEDING AND CURRENT ROW, rank() PARTITION BY [test.b, test.a] ORDER BY [test.c ASC NULLS FIRST] ROWS BETWEEN UNBOUNDED PRECEDING AND CURRENT ROW]]\
+            \n    TableScan: test";
+        assert_optimized_plan_eq(plan, expected)
+    }
+
     /// verifies that a filter is pushed to before a projection, the filter expression is correctly re-written
     #[test]
     fn alias() -> Result<()> {