From fa8ad401bed4df04c6d9e6745cd528b56d5820ec Mon Sep 17 00:00:00 2001
From: Liang-Chi Hsieh <viirya@gmail.com>
Date: Sun, 15 Jan 2023 13:12:39 -0800
Subject: [PATCH] Executing ProjectionExec with no column should not return an
 Err (#4912)

* Fix projection with no column

* Fix clippy
---
 .../core/src/physical_plan/projection.rs      | 27 +++++++++++++++++--
 1 file changed, 25 insertions(+), 2 deletions(-)

diff --git a/datafusion/core/src/physical_plan/projection.rs b/datafusion/core/src/physical_plan/projection.rs
index 14431b88951b..e418a8f93444 100644
--- a/datafusion/core/src/physical_plan/projection.rs
+++ b/datafusion/core/src/physical_plan/projection.rs
@@ -33,7 +33,7 @@ use crate::physical_plan::{
 };
 use arrow::datatypes::{Field, Schema, SchemaRef};
 use arrow::error::Result as ArrowResult;
-use arrow::record_batch::RecordBatch;
+use arrow::record_batch::{RecordBatch, RecordBatchOptions};
 use log::debug;
 
 use super::expressions::{Column, PhysicalSortExpr};
@@ -328,7 +328,13 @@ impl ProjectionStream {
             .map(|r| r.map(|v| v.into_array(batch.num_rows())))
             .collect::<Result<Vec<_>>>()?;
 
-        RecordBatch::try_new(self.schema.clone(), arrays)
+        if arrays.is_empty() {
+            let options =
+                RecordBatchOptions::new().with_row_count(Some(batch.num_rows()));
+            RecordBatch::try_new_with_options(self.schema.clone(), arrays, &options)
+        } else {
+            RecordBatch::try_new(self.schema.clone(), arrays)
+        }
     }
 }
 
@@ -372,6 +378,7 @@ impl RecordBatchStream for ProjectionStream {
 mod tests {
 
     use super::*;
+    use crate::physical_plan::common::collect;
     use crate::physical_plan::expressions::{self, col};
     use crate::prelude::SessionContext;
     use crate::scalar::ScalarValue;
@@ -418,6 +425,22 @@ mod tests {
         Ok(())
     }
 
+    #[tokio::test]
+    async fn project_no_column() -> Result<()> {
+        let session_ctx = SessionContext::new();
+        let task_ctx = session_ctx.task_ctx();
+
+        let csv = test::scan_partitioned_csv(1)?;
+        let expected = collect(csv.execute(0, task_ctx.clone())?).await.unwrap();
+
+        let projection = ProjectionExec::try_new(vec![], csv)?;
+        let stream = projection.execute(0, task_ctx.clone())?;
+        let output = collect(stream).await.unwrap();
+        assert_eq!(output.len(), expected.len());
+
+        Ok(())
+    }
+
     #[tokio::test]
     async fn test_stats_projection_columns_only() {
         let source = Statistics {