Fix COPY TO does not produce single output file for zero rows query

Author: Bert Vermeiren

datafusion/datasource/src/write/demux.rs

@@ -171,6 +171,22 @@ async fn row_count_demuxer(
         max_rows_per_file
     };
 
+    // Single-file output requires creating at least one file stream in advance.
+    // If no record batches are present in the input stream (zero-row scenario),
+    if single_file_output {
+        open_file_streams.push(create_new_file_stream(
+            &base_output_path,
+            &write_id,
+            part_idx,
+            &file_extension,
+            single_file_output,
+            max_buffered_batches,
+            &mut tx,
+        )?);
+        row_counts.push(0);
+        part_idx += 1;
+    }
+
     while let Some(rb) = input.next().await.transpose()? {
         // ensure we have at least minimum_parallel_files open
         if open_file_streams.len() < minimum_parallel_files {

datafusion/sqllogictest/test_files/copy.slt

@@ -426,6 +426,21 @@ select * from validate_parquet_single;
 1 Foo
 2 Bar
 
+# copy 0 rows to a single parquet file output
+query I
+COPY (SELECT 1 AS id WHERE FALSE) TO 'test_files/scratch/copy/table_no_rows.parquet';
+----
+0
+
+statement ok
+CREATE EXTERNAL TABLE validate_parquet_single_no_rows STORED AS PARQUET LOCATION 'test_files/scratch/copy/table_no_rows.parquet';
+
+# validate the parquet file contains 0 rows.
+query I
+SELECT COUNT(*) FROM validate_parquet_single_no_rows;
+----
+0
+
 # copy from table to folder of compressed json files
 query I
 COPY source_table  to 'test_files/scratch/copy/table_json_gz' STORED AS JSON OPTIONS ('format.compression' gzip);