[GitHub] [arrow-datafusion] alamb commented on a diff in pull request #2196: feat: Support ArrayIndex for ScalarValue(List)

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

alamb commented on code in PR #2196:
URL: https://github.com/apache/arrow-datafusion/pull/2196#discussion_r848396271


##########
datafusion/physical-expr/src/expressions/get_indexed_field.rs:
##########
@@ -105,9 +105,51 @@ impl PhysicalExpr for GetIndexedFieldExpr {
                 }
                 (dt, key) => Err(DataFusionError::NotImplemented(format!("get indexed field is only possible on lists with int64 indexes. Tried {} with {} index", dt, key))),
             },
-            ColumnarValue::Scalar(_) => Err(DataFusionError::NotImplemented(
-                "field access is not yet implemented for scalar values".to_string(),
-            )),
+            ColumnarValue::Scalar(scalar) => match (scalar.get_datatype(), &self.key) {
+                (DataType::List(v), ScalarValue::Int64(Some(i))) => {
+                    let wrapper = scalar.to_array();

Review Comment:
   I wonder if initially you might be able to use `ScalarValue::to_array_of_size` to convert the scalar argument into an `ArrayRef` and then use the same code as above:
   
   Another alternative might be to use the take kernel with something like
   
   Untested:
   
   ```rust
   ColumnarValue::Scalar(scalar) =>  {
     let indicies = self.key.to_array_of_size(array.len());
     let indicies = arrow::compute::cast(&indicies, DataType::Int32);
     let values = arrow::compute::take(indices, array)
   }
   ```
   
   Though I see the code above to handle non scalars is a bit more involved so perhaps `take` doesn't work -- though there is code for lists here: https://github.com/apache/arrow-rs/blob/master/arrow/src/compute/kernels/take.rs#L222-L228
   



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