From e27110fd11f6fb9a03e496b2f22e5a5adf883adb Mon Sep 17 00:00:00 2001
From: Andrew Lamb <andrew@nerdnetworks.org>
Date: Wed, 19 Jan 2022 07:09:31 -0500
Subject: [PATCH] Comment rationale for not directly comparing Dictionary types

---
 datafusion/src/physical_plan/coercion_rule/binary_rule.rs | 4 ++++
 datafusion/src/physical_plan/expressions/coercion.rs      | 6 +++++-
 2 files changed, 9 insertions(+), 1 deletion(-)

diff --git a/datafusion/src/physical_plan/coercion_rule/binary_rule.rs b/datafusion/src/physical_plan/coercion_rule/binary_rule.rs
index 984d0d5a4bd6..e616afb11ab1 100644
--- a/datafusion/src/physical_plan/coercion_rule/binary_rule.rs
+++ b/datafusion/src/physical_plan/coercion_rule/binary_rule.rs
@@ -77,6 +77,8 @@ pub(crate) fn coerce_types(
 }
 
 fn comparison_eq_coercion(lhs_type: &DataType, rhs_type: &DataType) -> Option<DataType> {
+    // can't compare dictionaries directly due to
+    // https://github.com/apache/arrow-rs/issues/1201
     if lhs_type == rhs_type && !is_dictionary(lhs_type) {
         // same type => equality is possible
         return Some(lhs_type.clone());
@@ -90,6 +92,8 @@ fn comparison_order_coercion(
     lhs_type: &DataType,
     rhs_type: &DataType,
 ) -> Option<DataType> {
+    // can't compare dictionaries directly due to
+    // https://github.com/apache/arrow-rs/issues/1201
     if lhs_type == rhs_type && !is_dictionary(lhs_type) {
         // same type => all good
         return Some(lhs_type.clone());
diff --git a/datafusion/src/physical_plan/expressions/coercion.rs b/datafusion/src/physical_plan/expressions/coercion.rs
index bcdfec0024db..7bfb567cbc7b 100644
--- a/datafusion/src/physical_plan/expressions/coercion.rs
+++ b/datafusion/src/physical_plan/expressions/coercion.rs
@@ -162,8 +162,10 @@ pub fn numerical_coercion(lhs_type: &DataType, rhs_type: &DataType) -> Option<Da
         return None;
     };
 
-    // same type => all good
+    // can't compare dictionaries directly due to
+    // https://github.com/apache/arrow-rs/issues/1201
     if lhs_type == rhs_type && !is_dictionary(lhs_type) {
+        // same type => all good
         return Some(lhs_type.clone());
     }
 
@@ -186,6 +188,8 @@ pub fn numerical_coercion(lhs_type: &DataType, rhs_type: &DataType) -> Option<Da
 
 // coercion rules for equality operations. This is a superset of all numerical coercion rules.
 pub fn eq_coercion(lhs_type: &DataType, rhs_type: &DataType) -> Option<DataType> {
+    // can't compare dictionaries directly due to
+    // https://github.com/apache/arrow-rs/issues/1201
     if lhs_type == rhs_type && !is_dictionary(lhs_type) {
         // same type => equality is possible
         return Some(lhs_type.clone());