non-strict cast should always block predicate pushdown on that col (#…

…2607)

polars/polars-lazy/src/logical_plan/optimizer/predicate_pushdown/utils.rs

@@ -119,6 +119,8 @@ pub(super) fn other_column_is_pushdown_boundary(node: Node, expr_arena: &Arena<A
             | AExpr::Function {options: FunctionOptions { collect_groups: ApplyOptions::ApplyList, .. }, ..}
             | AExpr::BinaryExpr {..}
             | AExpr::Cast {data_type: DataType::Float32 | DataType::Float64, ..}
+            // cast may create nulls
+            | AExpr::Cast {strict: false, ..}
             // still need to investigate this one
             | AExpr::Explode {..}
             // A groupby needs all rows for aggregation

polars/polars-lazy/src/tests/predicate_queries.rs

@@ -196,3 +196,25 @@ fn test_filter_nulls_created_by_join() -> Result<()> {
 
     Ok(())
 }
+
+#[test]
+fn test_filter_null_creation_by_cast() -> Result<()> {
+    let df = df![
+        "int" => [1, 2, 3],
+        "empty" => ["", "", ""]
+    ]?;
+
+    let out = df
+        .lazy()
+        .with_column(col("empty").cast(DataType::Int32).alias("empty"))
+        .filter(col("empty").is_null().and(col("int").eq(lit(3i32))))
+        .collect()?;
+
+    let expected = df![
+        "int" => [3],
+        "empty" => &[None, Some(1i32)][..1]
+    ]?;
+    assert!(out.frame_equal_missing(&expected));
+
+    Ok(())
+}