lazy; empty projection should give empty dataframe

polars/polars-lazy/src/logical_plan/mod.rs

@@ -933,16 +933,19 @@ impl LogicalPlanBuilder {
     pub fn project(self, exprs: Vec<Expr>) -> Self {
         let (exprs, schema) = prepare_projection(exprs, self.0.schema());
 
-        // if len == 0, no projection has to be done. This is a select all operation.
-        if !exprs.is_empty() {
+        if exprs.is_empty() {
+            self.map(
+                |_| Ok(DataFrame::new_no_checks(vec![])),
+                AllowedOptimizations::default(),
+                Some(Arc::new(Schema::default())),
+            )
+        } else {
             LogicalPlan::Projection {
                 expr: exprs,
                 input: Box::new(self.0),
                 schema: Arc::new(schema),
             }
             .into()
-        } else {
-            self
         }
     }
 

py-polars/tests/test_df.py

@@ -1325,3 +1325,7 @@ def test_df_schema_unique() -> None:
 
     with pytest.raises(Exception):
         df.rename({"b": "a"})
+
+
+def test_empty_projection() -> None:
+    assert pl.DataFrame({"a": [1, 2], "b": [3, 4]}).select([]).shape == (0, 0)