fix: handle zero-column DuckDB sources

Author: alibro005

src/freshdata/execution/backends/_duckdb.py

@@ -42,14 +42,14 @@
 )
 from .._plan import PlanGenerator
 from .._report import finalize_report, finalize_report_native, init_report, zero_column_frame
+from ...report import CleanReport
 from .._spill import create_run_spill_dir, remove_run_spill_dir
 from ._pandas import materialize_to_pandas
 
 if TYPE_CHECKING:  # pragma: no cover - typing only
     import pandas as pd
 
     from ...config import CleanConfig
-    from ...report import CleanReport
     from .._config import EngineConfig
 
 log = logging.getLogger("freshdata.execution.duckdb")
@@ -147,6 +147,19 @@ def execute(
 
         plan_cols = self._peek_columns(source)
         plan = PlanGenerator(config).plan(plan_cols)
+
+        if not plan_cols:
+            started = time.perf_counter()
+            report = CleanReport(
+                rows_before=len(source),
+                cols_before=0,
+                memory_before=self._memory_before(source),
+            )
+            report.backend = "duckdb"
+            cleaned = zero_column_frame("duckdb", engine_config.output_format, report)
+            finalize_report(report, cleaned, started)
+            return cleaned, report
+
         reason = plan.fallback_reason or pandas_ingest_fallback_reason(source, self.name)
         if reason is None and self._pandas_index_forces_fallback(source):
             reason = "pandas index semantics"

tests/test_execution/test_duckdb_engine.py

@@ -52,6 +52,12 @@ def test_drop_empty_column(native_config):
     assert "empty" not in out.columns
 
 
+def test_zero_column_dataframe(native_config):
+    df = pd.DataFrame(index=range(3))
+    out = fd.clean(df, config=native_config, engine="duckdb")
+    assert out.shape == (3, 0)
+
+
 def test_drop_duplicates(native_config):
     df = pd.DataFrame({"a": [1, 1, 2, 2, 3], "b": ["x", "x", "y", "y", "z"]})
     out = fd.clean(df, config=native_config, engine="duckdb", drop_duplicates=True)