Add Expr::column_refs to find column references without copying

datafusion/expr/src/expr.rs

@@ -33,7 +33,9 @@ use crate::{
 use crate::{window_frame, Volatility};
 
 use arrow::datatypes::{DataType, FieldRef};
-use datafusion_common::tree_node::{Transformed, TransformedResult, TreeNode};
+use datafusion_common::tree_node::{
+    Transformed, TransformedResult, TreeNode, TreeNodeRecursion,
+};
 use datafusion_common::{
     internal_err, plan_err, Column, DFSchema, Result, ScalarValue, TableReference,
 };
@@ -1333,6 +1335,46 @@ impl Expr {
         Ok(using_columns)
     }
 
+    /// Return all references to columns in this expression.
+    ///
+    /// # Example
+    /// ```
+    /// # use std::collections::HashSet;
+    /// # use datafusion_common::Column;
+    /// # use datafusion_expr::col;
+    /// // For an expression `a + (b * a)`
+    /// let expr = col("a") + (col("b") * col("a"));
+    /// let refs = expr.column_refs();
+    /// // refs contains "a" and "b"
+    /// assert_eq!(refs.len(), 2);
+    /// assert!(refs.contains(&Column::new_unqualified("a")));
+    ///  assert!(refs.contains(&Column::new_unqualified("b")));
+    /// ```
+    pub fn column_refs(&self) -> HashSet<&Column> {
+        let mut using_columns = HashSet::new();
+        self.add_column_refs(&mut using_columns);
+        using_columns
+    }
+
+    /// Adds references to all columns in this expression to the set
+    ///
+    /// See [`Self::column_refs`] for details
+    pub fn add_column_refs<'a>(&'a self, set: &mut HashSet<&'a Column>) {
+        self.apply(|expr| {
+            if let Expr::Column(col) = expr {
+                set.insert(col);
+            }
+            Ok(TreeNodeRecursion::Continue)
+        })
+        .expect("traversal is infallable");
+    }
+
+    /// Returns true if there are any column references in this Expr
+    pub fn any_column_refs(&self) -> bool {
+        self.exists(|expr| Ok(matches!(expr, Expr::Column(_))))
+            .unwrap()
+    }
+
     /// Return true when the expression contains out reference(correlated) expressions.
     pub fn contains_outer(&self) -> bool {
         self.exists(|expr| Ok(matches!(expr, Expr::OuterReferenceColumn { .. })))
@@ -2038,15 +2080,15 @@ mod test {
         // single column
         {
             let expr = &Expr::Cast(Cast::new(Box::new(col("a")), DataType::Float64));
-            let columns = expr.to_columns()?;
+            let columns = expr.column_refs();
             assert_eq!(1, columns.len());
             assert!(columns.contains(&Column::from_name("a")));
         }
 
         // multiple columns
         {
             let expr = col("a") + col("b") + lit(1);
-            let columns = expr.to_columns()?;
+            let columns = expr.column_refs();
             assert_eq!(2, columns.len());
             assert!(columns.contains(&Column::from_name("a")));
             assert!(columns.contains(&Column::from_name("b")));

datafusion/expr/src/utils.rs

@@ -46,6 +46,7 @@ pub const COUNT_STAR_EXPANSION: ScalarValue = ScalarValue::Int64(Some(1));
 
 /// Recursively walk a list of expression trees, collecting the unique set of columns
 /// referenced in the expression
+#[deprecated(since = "40.0.0", note = "Expr::add_column_refs instead")]
 pub fn exprlist_to_columns(expr: &[Expr], accum: &mut HashSet<Column>) -> Result<()> {
     for e in expr {
         expr_to_columns(e, accum)?;

datafusion/optimizer/src/analyzer/subquery.rs

@@ -300,19 +300,17 @@ fn can_pullup_over_aggregation(expr: &Expr) -> bool {
     }) = expr
     {
         match (left.deref(), right.deref()) {
-            (Expr::Column(_), right) if right.to_columns().unwrap().is_empty() => true,
-            (left, Expr::Column(_)) if left.to_columns().unwrap().is_empty() => true,
+            (Expr::Column(_), right) => !right.any_column_refs(),
+            (left, Expr::Column(_)) => !left.any_column_refs(),
             (Expr::Cast(Cast { expr, .. }), right)
-                if matches!(expr.deref(), Expr::Column(_))
-                    && right.to_columns().unwrap().is_empty() =>
+                if matches!(expr.deref(), Expr::Column(_)) =>
             {
-                true
+                !right.any_column_refs()
             }
             (left, Expr::Cast(Cast { expr, .. }))
-                if matches!(expr.deref(), Expr::Column(_))
-                    && left.to_columns().unwrap().is_empty() =>
+                if matches!(expr.deref(), Expr::Column(_)) =>
             {
-                true
+                !left.any_column_refs()
             }
             (_, _) => false,
         }
@@ -323,9 +321,10 @@ fn can_pullup_over_aggregation(expr: &Expr) -> bool {
 
 /// Check whether the window expressions contain a mixture of out reference columns and inner columns
 fn check_mixed_out_refer_in_window(window: &Window) -> Result<()> {
-    let mixed = window.window_expr.iter().any(|win_expr| {
-        win_expr.contains_outer() && !win_expr.to_columns().unwrap().is_empty()
-    });
+    let mixed = window
+        .window_expr
+        .iter()
+        .any(|win_expr| win_expr.contains_outer() && win_expr.any_column_refs());
     if mixed {
         plan_err!(
             "Window expressions should not contain a mixed of outer references and inner columns"

datafusion/optimizer/src/decorrelate.rs

@@ -370,11 +370,14 @@ impl PullUpCorrelatedExpr {
             }
         }
         if let Some(pull_up_having) = &self.pull_up_having_expr {
-            let filter_apply_columns = pull_up_having.to_columns()?;
+            let filter_apply_columns = pull_up_having.column_refs();
             for col in filter_apply_columns {
-                let col_expr = Expr::Column(col);
-                if !missing_exprs.contains(&col_expr) {
-                    missing_exprs.push(col_expr)
+                // add to missing_exprs if not already there
+                let contains = missing_exprs
+                    .iter()
+                    .any(|expr| matches!(expr, Expr::Column(c) if c == col));
+                if !contains {
+                    missing_exprs.push(Expr::Column(col.clone()))
                 }
             }
         }

datafusion/optimizer/src/optimize_projections/mod.rs

@@ -479,10 +479,10 @@ fn merge_consecutive_projections(proj: Projection) -> Result<Transformed<Project
     };
 
     // Count usages (referrals) of each projection expression in its input fields:
-    let mut column_referral_map = HashMap::<Column, usize>::new();
-    for columns in expr.iter().flat_map(|expr| expr.to_columns()) {
+    let mut column_referral_map = HashMap::<&Column, usize>::new();
+    for columns in expr.iter().map(|expr| expr.column_refs()) {
         for col in columns.into_iter() {
-            *column_referral_map.entry(col.clone()).or_default() += 1;
+            *column_referral_map.entry(col).or_default() += 1;
         }
     }
 
@@ -493,7 +493,7 @@ fn merge_consecutive_projections(proj: Projection) -> Result<Transformed<Project
         usage > 1
             && !is_expr_trivial(
                 &prev_projection.expr
-                    [prev_projection.schema.index_of_column(&col).unwrap()],
+                    [prev_projection.schema.index_of_column(col).unwrap()],
             )
     }) {
         // no change
@@ -625,12 +625,12 @@ fn rewrite_expr(expr: Expr, input: &Projection) -> Result<Transformed<Expr>> {
 /// * `expr` - The expression to analyze for outer-referenced columns.
 /// * `columns` - A mutable reference to a `HashSet<Column>` where detected
 ///   columns are collected.
-fn outer_columns(expr: &Expr, columns: &mut HashSet<Column>) {
+fn outer_columns<'a>(expr: &'a Expr, columns: &mut HashSet<&'a Column>) {
     // inspect_expr_pre doesn't handle subquery references, so find them explicitly
     expr.apply(|expr| {
         match expr {
             Expr::OuterReferenceColumn(_, col) => {
-                columns.insert(col.clone());
+                columns.insert(col);
             }
             Expr::ScalarSubquery(subquery) => {
                 outer_columns_helper_multi(&subquery.outer_ref_columns, columns);
@@ -660,9 +660,9 @@ fn outer_columns(expr: &Expr, columns: &mut HashSet<Column>) {
 /// * `exprs` - The expressions to analyze for outer-referenced columns.
 /// * `columns` - A mutable reference to a `HashSet<Column>` where detected
 ///   columns are collected.
-fn outer_columns_helper_multi<'a>(
+fn outer_columns_helper_multi<'a, 'b>(
     exprs: impl IntoIterator<Item = &'a Expr>,
-    columns: &mut HashSet<Column>,
+    columns: &'b mut HashSet<&'a Column>,
 ) {
     exprs.into_iter().for_each(|e| outer_columns(e, columns));
 }

datafusion/optimizer/src/optimize_projections/required_indices.rs

@@ -113,12 +113,12 @@ impl RequiredIndicies {
     /// * `expr`: An expression for which we want to find necessary field indices.
     fn add_expr(&mut self, input_schema: &DFSchemaRef, expr: &Expr) -> Result<()> {
         // TODO could remove these clones (and visit the expression directly)
-        let mut cols = expr.to_columns()?;
+        let mut cols = expr.column_refs();
         // Get outer-referenced (subquery) columns:
         outer_columns(expr, &mut cols);
         self.indices.reserve(cols.len());
         for col in cols {
-            if let Some(idx) = input_schema.maybe_index_of_column(&col) {
+            if let Some(idx) = input_schema.maybe_index_of_column(col) {
                 self.indices.push(idx);
             }
         }

datafusion/optimizer/src/push_down_filter.rs

@@ -561,12 +561,9 @@ fn infer_join_predicates(
         .filter_map(|predicate| {
             let mut join_cols_to_replace = HashMap::new();
 
-            let columns = match predicate.to_columns() {
-                Ok(columns) => columns,
-                Err(e) => return Some(Err(e)),
-            };
+            let columns = predicate.column_refs();
 
-            for col in columns.iter() {
+            for &col in columns.iter() {
                 for (l, r) in join_col_keys.iter() {
                     if col == *l {
                         join_cols_to_replace.insert(col, *r);
@@ -798,7 +795,7 @@ impl OptimizerRule for PushDownFilter {
                 let mut keep_predicates = vec![];
                 let mut push_predicates = vec![];
                 for expr in predicates {
-                    let cols = expr.to_columns()?;
+                    let cols = expr.column_refs();
                     if cols.iter().all(|c| group_expr_columns.contains(c)) {
                         push_predicates.push(expr);
                     } else {
@@ -899,7 +896,7 @@ impl OptimizerRule for PushDownFilter {
                 let predicate_push_or_keep = split_conjunction(&filter.predicate)
                     .iter()
                     .map(|expr| {
-                        let cols = expr.to_columns()?;
+                        let cols = expr.column_refs();
                         if cols.iter().any(|c| prevent_cols.contains(&c.name)) {
                             Ok(false) // No push (keep)
                         } else {

datafusion/optimizer/src/utils.rs

@@ -72,9 +72,9 @@ pub(crate) fn collect_subquery_cols(
 ) -> Result<BTreeSet<Column>> {
     exprs.iter().try_fold(BTreeSet::new(), |mut cols, expr| {
         let mut using_cols: Vec<Column> = vec![];
-        for col in expr.to_columns()?.into_iter() {
-            if subquery_schema.has_column(&col) {
-                using_cols.push(col);
+        for col in expr.column_refs().into_iter() {
+            if subquery_schema.has_column(col) {
+                using_cols.push(col.clone());
             }
         }
 

datafusion/sql/src/statement.rs

@@ -964,7 +964,7 @@ impl<'a, S: ContextProvider> SqlToRel<'a, S> {
                 self.order_by_to_sort_expr(&expr, schema, planner_context, true, None)?;
             // Verify that columns of all SortExprs exist in the schema:
             for expr in expr_vec.iter() {
-                for column in expr.to_columns()?.iter() {
+                for column in expr.column_refs().iter() {
                     if !schema.has_column(column) {
                         // Return an error if any column is not in the schema:
                         return plan_err!("Column {column} is not in schema");