Only push down unstrict casts

Author: evertlammerts

duckdb/polars_io.py

@@ -162,6 +162,9 @@ def _pl_tree_to_sql(tree: _ExpressionTree) -> str:
     if node_type == "Cast":
         cast_tree = tree[node_type]
         assert isinstance(cast_tree, dict), f"A {node_type} should be a dict but got {type(cast_tree)}"
+        if cast_tree.get("options") != "NonStrict":
+            msg = f"Only NonStrict casts can be safely unwrapped, got {cast_tree.get('options')!r}"
+            raise NotImplementedError(msg)
         cast_expr = cast_tree["expr"]
         assert isinstance(cast_expr, dict), f"A {node_type} should be a dict but got {type(cast_expr)}"
         return _pl_tree_to_sql(cast_expr)

tests/fast/arrow/test_polars.py

@@ -739,3 +739,9 @@ def test_polars_lazy_pushdown_decimal_with_cast(self):
 
         assert lazy_df.filter(pl.col("a") == 1).collect().to_dicts() == [{"a": 1}]
         assert lazy_df.filter(pl.col("a") > 1).collect().to_dicts() == [{"a": 10}, {"a": 100}]
+
+    def test_explicit_cast_not_pushed_down(self):
+        """Explicit user .cast() (Strict) should not be pushed down - falls back to Polars."""
+        # pl.col("a").cast(pl.Int64) produces a Strict Cast node
+        expr = pl.col("a").cast(pl.Int64) > 5
+        invalid_filter(expr)