feat: Expand NDJson glob into one SCAN

crates/polars-lazy/src/scan/ndjson.rs

@@ -91,6 +91,44 @@ impl LazyJsonLineReader {
 }
 
 impl LazyFileListReader for LazyJsonLineReader {
+    fn finish(self) -> PolarsResult<LazyFrame> {
+        if !self.glob() {
+            return self.finish_no_glob();
+        }
+
+        let paths = self.expand_paths()?.0;
+
+        let file_options = FileScanOptions {
+            n_rows: self.n_rows,
+            with_columns: None,
+            cache: false,
+            row_index: self.row_index,
+            rechunk: self.rechunk,
+            file_counter: 0,
+            hive_options: Default::default(),
+        };
+
+        let options = NDJsonReadOptions {
+            n_threads: None,
+            infer_schema_length: self.infer_schema_length,
+            chunk_size: NonZeroUsize::new(1 << 18).unwrap(),
+            low_memory: self.low_memory,
+            ignore_errors: self.ignore_errors,
+            schema: self.schema,
+        };
+
+        let scan_type = FileScan::NDJson { options };
+
+        Ok(LazyFrame::from(DslPlan::Scan {
+            paths,
+            file_info: None,
+            hive_parts: None,
+            predicate: None,
+            file_options,
+            scan_type,
+        }))
+    }
+
     fn finish_no_glob(self) -> PolarsResult<LazyFrame> {
         let file_options = FileScanOptions {
             n_rows: self.n_rows,

crates/polars-mem-engine/src/executors/scan/ndjson.rs

@@ -35,28 +35,50 @@ impl JsonExec {
             .as_ref()
             .unwrap_right();
 
+        let mut n_rows = self.file_scan_options.n_rows;
+
         let dfs = self
             .paths
             .iter()
-            .map(|p| {
-                let df = JsonLineReader::from_path(p)?
+            .map_while(|p| {
+                if n_rows == Some(0) {
+                    return None;
+                }
+
+                let reader = match JsonLineReader::from_path(p) {
+                    Ok(r) => r,
+                    Err(e) => return Some(Err(e)),
+                };
+
+                let df = reader
                     .with_schema(schema.clone())
                     .with_rechunk(self.file_scan_options.rechunk)
                     .with_chunk_size(Some(self.options.chunk_size))
                     .low_memory(self.options.low_memory)
-                    .with_n_rows(self.file_scan_options.n_rows)
+                    .with_n_rows(n_rows)
                     .with_ignore_errors(self.options.ignore_errors)
-                    .finish()?;
+                    .finish();
 
-                if let Some(row_index) = &mut self.file_scan_options.row_index {
-                    let offset = row_index.offset;
-                    row_index.offset += df.height() as IdxSize;
-                    df.with_row_index(row_index.name.as_ref(), Some(offset))
-                } else {
-                    Ok(df)
+                let df = match df {
+                    Ok(df) => df,
+                    Err(e) => return Some(Err(e)),
+                };
+
+                if let Some(ref mut n_rows) = n_rows {
+                    *n_rows -= df.height();
                 }
+
+                Some(match self.file_scan_options.row_index {
+                    Some(ref mut row_index) => {
+                        let offset = row_index.offset;
+                        row_index.offset += df.height() as IdxSize;
+                        df.with_row_index(row_index.name.as_ref(), Some(offset))
+                    },
+                    None => Ok(df),
+                })
             })
             .collect::<PolarsResult<Vec<_>>>()?;
+
         accumulate_dataframes_vertical(dfs)
     }
 }

py-polars/tests/unit/io/test_lazy_json.py

@@ -138,3 +138,13 @@ def test_ndjson_list_arg(io_files_path: Path) -> None:
     assert df.shape == (54, 4)
     assert df.row(-1) == ("seafood", 194, 12.0, 1)
     assert df.row(0) == ("vegetables", 45, 0.5, 2)
+
+
+def test_glob_single_scan(io_files_path: Path) -> None:
+    file_path = io_files_path / "foods*.ndjson"
+    df = pl.scan_ndjson(file_path, n_rows=40)
+
+    explain = df.explain()
+
+    assert explain.count("SCAN") == 1
+    assert "UNION" not in explain