fix: make PushDownLeafProjections work with unnest

datafusion/optimizer/src/extract_leaf_expressions.rs

@@ -1143,6 +1143,13 @@ fn try_push_into_inputs(
         return Ok(None);
     }
 
+    // Unnest may output a column with the same name but different value/type
+    // than its input column. Name-based routing cannot distinguish those.
+    // On top of that Unnest can't go through the `node.with_new_exprs(node.expressions(), new_inputs)` rebuild
+    if matches!(node, LogicalPlan::Unnest(_)) {
+        return Ok(None);
+    }
+
     // SubqueryAlias remaps qualifiers between input and output.
     // Rewrite pairs/columns from alias-space to input-space before routing.
     let remapped = if let LogicalPlan::SubqueryAlias(sa) = node {
@@ -3035,4 +3042,48 @@ mod tests {
 
         Ok(())
     }
+
+    /// Regression test for the `Assertion failed: expr.is_empty(): Unnest`
+    /// internal error.
+    ///
+    /// `try_push_into_inputs` rebuilds the parent node via
+    /// `node.with_new_exprs(node.expressions(), new_inputs)`. For `Unnest`,
+    /// `apply_expressions` exposes the `exec_columns` as `Expr::Column`s
+    /// (so `expressions()` is **non-empty**), but `with_new_exprs` for
+    /// `Unnest` immediately calls `assert_no_expressions(expr)?` and errors
+    /// out. The optimizer should treat `Unnest` as a barrier and bail
+    /// instead of attempting to push through it.
+    #[test]
+    fn test_no_push_through_unnest() -> Result<()> {
+        use arrow::datatypes::{DataType, Field, Schema};
+
+        let schema = Schema::new(vec![
+            Field::new("list_col", DataType::new_list(DataType::Int32, true), true),
+            Field::new("other_col", DataType::Int32, true),
+        ]);
+        let table_scan =
+            datafusion_expr::logical_plan::table_scan(Some("t"), &schema, None)?
+                .build()?;
+        let plan = LogicalPlanBuilder::from(table_scan)
+            .unnest_column("list_col")?
+            .filter(leaf_udf(col("list_col"), "x").eq(lit(1i32)))?
+            .build()?;
+
+        let ctx = OptimizerContext::new().with_max_passes(1);
+        let optimizer = Optimizer::with_rules(vec![
+            Arc::new(ExtractLeafExpressions::new()),
+            Arc::new(PushDownLeafProjections::new()),
+        ]);
+        let optimized = optimizer.optimize(plan, &ctx, |_, _| {})?;
+
+        insta::assert_snapshot!(format!("{optimized}"), @r#"
+        Projection: list_col, t.other_col
+          Filter: __datafusion_extracted_1 = Int32(1)
+            Projection: leaf_udf(list_col, Utf8("x")) AS __datafusion_extracted_1, list_col, t.other_col
+              Unnest: lists[t.list_col|depth=1] structs[]
+                TableScan: t
+        "#);
+
+        Ok(())
+    }
 }

datafusion/sqllogictest/test_files/unnest.slt

@@ -1419,3 +1419,47 @@ FROM (
 
 statement ok
 DROP TABLE unused_unnest_pruning;
+
+## Regression: pushing a leaf-extracted projection (containing get_field,
+## which has MoveTowardsLeafNodes placement) through an `Unnest` used to
+## trip `Assertion failed: expr.is_empty(): Unnest` inside
+## `PushDownLeafProjections`. The optimizer must not try to pushdown these 
+## projections through an `Unnest` and should produce a valid plan.
+
+statement ok
+CREATE TABLE struct_and_list_table
+AS VALUES
+    (struct(1, 2), [10, 20, 30]),
+    (struct(3, 4), [40, 50]);
+
+query I
+SELECT sum(get_field(s, 'c0'))
+FROM (SELECT s, unnest(arr)
+      FROM (SELECT column1 AS s, column2 AS arr
+            FROM struct_and_list_table));
+----
+9
+
+statement ok
+DROP TABLE struct_and_list_table;
+
+## Regression: get_field directly references the struct produced by unnest.
+## This covers the case where the leaf-extracted expression depends on the
+## unnested column itself rather than a sibling input column below the Unnest.
+
+statement ok
+CREATE TABLE list_struct_table
+AS VALUES
+    ([struct(1, 'a'), struct(2, 'b')]),
+    ([struct(3, 'c')]);
+
+query IT
+SELECT get_field(unnest(column1), 'c0'), get_field(unnest(column1), 'c1')
+FROM list_struct_table;
+----
+1 a
+2 b
+3 c
+
+statement ok
+DROP TABLE list_struct_table;