[GitHub] [arrow-datafusion] alamb commented on a change in pull request #1726: API to get Expr's type and nullability without a `DFSchema`

[GitBox <gi...@apache.org>]

alamb commented on a change in pull request #1726:
URL: https://github.com/apache/arrow-datafusion/pull/1726#discussion_r798488745



##########
File path: datafusion/src/logical_plan/expr.rs
##########
@@ -392,20 +392,54 @@ impl PartialOrd for Expr {
     }
 }
 
+/// Provides schema information needed by [Expr] methods such as
+/// [Expr::nullable] and [Expr::data_type]
+///
+/// Note that this trait is implemented for &[DFSchema] which is
+/// widely used in the DataFusion codebase.
+pub trait ExprSchema {
+    /// Is this column reference nullable?
+    fn nullable(&self, col: &Column) -> Result<bool>;
+
+    /// What is the datatype of this column?
+    fn data_type(&self, col: &Column) -> Result<&DataType>;
+}
+
+// Implement for Arc<DFSchema>
+impl<P: AsRef<DFSchema>> ExprSchema for P {
+    fn nullable(&self, col: &Column) -> Result<bool> {
+        self.as_ref().nullable(col)
+    }
+
+    fn data_type(&self, col: &Column) -> Result<&DataType> {
+        self.as_ref().data_type(col)
+    }
+}
+
+impl ExprSchema for DFSchema {
+    fn nullable(&self, col: &Column) -> Result<bool> {
+        Ok(self.field_from_column(col)?.is_nullable())
+    }
+
+    fn data_type(&self, col: &Column) -> Result<&DataType> {
+        Ok(self.field_from_column(col)?.data_type())
+    }
+}
+
 impl Expr {
-    /// Returns the [arrow::datatypes::DataType] of the expression based on [arrow::datatypes::Schema].
+    /// Returns the [arrow::datatypes::DataType] of the expression based on [DFSchema].

Review comment:
       good point -- will fix




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