skip iterating if there are no row groups after prune

crates/polars-io/src/parquet/read.rs

@@ -367,15 +367,22 @@ impl ParquetAsyncReader {
 
     pub async fn finish(mut self) -> PolarsResult<DataFrame> {
         let rechunk = self.rechunk;
-        let metadata = self.get_metadata().await?.clone();
         let reader_schema = self.schema().await?;
         let row_index = self.row_index.clone();
         let hive_partition_columns = self.hive_partition_columns.clone();
         let projection = self.projection.clone();
 
         // batched reader deals with slice pushdown
         let reader = self.batched(usize::MAX).await?;
-        let n_batches = metadata.row_groups.len();
+        let n_batches = reader.num_row_groups();
+        if n_batches == 0 {
+            return Ok(materialize_empty_df(
+                projection.as_deref(),
+                reader_schema.as_ref(),
+                hive_partition_columns.as_deref(),
+                row_index.as_ref(),
+            ));
+        }
         let mut iter = reader.iter(n_batches);
 
         let mut chunks = Vec::with_capacity(n_batches);

crates/polars-io/src/parquet/read_impl.rs

@@ -578,6 +578,10 @@ impl BatchedParquetReader {
         self.limit == 0
     }
 
+    pub fn num_row_groups(&self) -> usize {
+        self.row_group_metadata.len()
+    }
+
     pub fn schema(&self) -> &ArrowSchemaRef {
         &self.schema
     }