alamb commented on a change in pull request #8400:
URL: https://github.com/apache/arrow/pull/8400#discussion_r501795062
##########
File path: rust/datafusion/src/logical_plan/mod.rs
##########
@@ -323,12 +324,13 @@ impl Expr {
///
/// # Errors
///
- /// This function errors when it is impossible to cast the expression to
the target [arrow::datatypes::DataType].
+ /// This function errors when it is impossible to cast the
+ /// expression to the target [arrow::datatypes::DataType].
pub fn cast_to(&self, cast_to_type: &DataType, schema: &Schema) ->
Result<Expr> {
let this_type = self.get_type(schema)?;
if this_type == *cast_to_type {
Ok(self.clone())
- } else if can_coerce_from(cast_to_type, &this_type) {
+ } else if can_cast_types(&this_type, cast_to_type) {
Review comment:
the idea is to use a common `can_cast_types` function to detect valid
casts at plan time, and make `can_cast_types` consistent with the arrow cast
kernel
##########
File path: rust/datafusion/src/physical_plan/type_casting.rs
##########
@@ -0,0 +1,218 @@
+// Licensed to the Apache Software Foundation (ASF) under one
+// or more contributor license agreements. See the NOTICE file
+// distributed with this work for additional information
+// regarding copyright ownership. The ASF licenses this file
+// to you under the Apache License, Version 2.0 (the
+// "License"); you may not use this file except in compliance
+// with the License. You may obtain a copy of the License at
+//
+// http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing,
+// software distributed under the License is distributed on an
+// "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+// KIND, either express or implied. See the License for the
+// specific language governing permissions and limitations
+// under the License.
+
+//! This module provides a way of checking what type casts are
+//! supported at planning time for DataFusion. Since DataFusion uses
+//! the Arrow `cast` compute kernel, the supported casts are the same
+//! as the Arrow casts.
+//!
+//! The rules in this module are designed to be redundant with the
+//! rules in the Arrow `cast` kernel. The redundancy is needed so that
+//! DataFusion can generate an error at plan time rather than during
+//! execution (which could happen many hours after execution starts,
+//! when the query finally reaches that point)
+//!
+
+use arrow::datatypes::*;
+
+/// Return true if a value of type `from_type` can be cast into a
+/// value of `to_type`. Note that such as cast may be lossy. For
+/// lossless type conversions, see the `type_coercion` module
+///
+/// See the module level documentation for more detail on casting
+pub fn can_cast_types(from_type: &DataType, to_type: &DataType) -> bool {
+ use self::DataType::*;
+ if from_type == to_type {
+ return true;
+ }
+
+ // Note this is meant to mirror the structure in
arrow/src/compute/kernels/cast.rs
+ match (from_type, to_type) {
Review comment:
@jorgecarleitao -- I think the idea of using a single `match` in
cast.rs is a good one. I'll try and whip up a proposed PR with that pattern for
discussion (though probably not until tomorrow morning US Eastern time, or the
weekend)
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