fix: Add dictionary coercion support for numeric comparison operations

datafusion/expr-common/src/type_coercion/binary.rs

@@ -866,6 +866,7 @@ pub fn comparison_coercion_numeric(
         return Some(lhs_type.clone());
     }
     binary_numeric_coercion(lhs_type, rhs_type)
+        .or_else(|| dictionary_comparison_coercion_numeric(lhs_type, rhs_type, true))
         .or_else(|| string_coercion(lhs_type, rhs_type))
         .or_else(|| null_coercion(lhs_type, rhs_type))
         .or_else(|| string_numeric_coercion_as_numeric(lhs_type, rhs_type))
@@ -1353,38 +1354,75 @@ fn both_numeric_or_null_and_numeric(lhs_type: &DataType, rhs_type: &DataType) ->
     }
 }
 
-/// Coercion rules for Dictionaries: the type that both lhs and rhs
+/// Generic coercion rules for Dictionaries: the type that both lhs and rhs
 /// can be casted to for the purpose of a computation.
 ///
 /// Not all operators support dictionaries, if `preserve_dictionaries` is true
-/// dictionaries will be preserved if possible
-fn dictionary_comparison_coercion(
+/// dictionaries will be preserved if possible.
+///
+/// The `coerce_fn` parameter determines which comparison coercion function to use
+/// for comparing the dictionary value types.
+fn dictionary_comparison_coercion_generic(
     lhs_type: &DataType,
     rhs_type: &DataType,
     preserve_dictionaries: bool,
+    coerce_fn: fn(&DataType, &DataType) -> Option<DataType>,
 ) -> Option<DataType> {
     use arrow::datatypes::DataType::*;
     match (lhs_type, rhs_type) {
         (
             Dictionary(_lhs_index_type, lhs_value_type),
             Dictionary(_rhs_index_type, rhs_value_type),
-        ) => comparison_coercion(lhs_value_type, rhs_value_type),
+        ) => coerce_fn(lhs_value_type, rhs_value_type),
         (d @ Dictionary(_, value_type), other_type)
         | (other_type, d @ Dictionary(_, value_type))
             if preserve_dictionaries && value_type.as_ref() == other_type =>
         {
             Some(d.clone())
         }
-        (Dictionary(_index_type, value_type), _) => {
-            comparison_coercion(value_type, rhs_type)
-        }
-        (_, Dictionary(_index_type, value_type)) => {
-            comparison_coercion(lhs_type, value_type)
-        }
+        (Dictionary(_index_type, value_type), _) => coerce_fn(value_type, rhs_type),
+        (_, Dictionary(_index_type, value_type)) => coerce_fn(lhs_type, value_type),
         _ => None,
     }
 }
 
+/// Coercion rules for Dictionaries: the type that both lhs and rhs
+/// can be casted to for the purpose of a computation.
+///
+/// Not all operators support dictionaries, if `preserve_dictionaries` is true
+/// dictionaries will be preserved if possible
+fn dictionary_comparison_coercion(
+    lhs_type: &DataType,
+    rhs_type: &DataType,
+    preserve_dictionaries: bool,
+) -> Option<DataType> {
+    dictionary_comparison_coercion_generic(
+        lhs_type,
+        rhs_type,
+        preserve_dictionaries,
+        comparison_coercion,
+    )
+}
+
+/// Coercion rules for Dictionaries with numeric preference: similar to
+/// [`dictionary_comparison_coercion`] but uses [`comparison_coercion_numeric`]
+/// which prefers numeric types over strings when both are present.
+///
+/// This is used by [`comparison_coercion_numeric`] to maintain consistent
+/// numeric-preferring semantics when dealing with dictionary types.
+fn dictionary_comparison_coercion_numeric(
+    lhs_type: &DataType,
+    rhs_type: &DataType,
+    preserve_dictionaries: bool,
+) -> Option<DataType> {
+    dictionary_comparison_coercion_generic(
+        lhs_type,
+        rhs_type,
+        preserve_dictionaries,
+        comparison_coercion_numeric,
+    )
+}
+
 /// Coercion rules for string concat.
 /// This is a union of string coercion rules and specified rules:
 /// 1. At least one side of lhs and rhs should be string type (Utf8 / LargeUtf8)

datafusion/sqllogictest/test_files/nullif.slt

@@ -174,3 +174,39 @@ query T
 SELECT NULLIF(arrow_cast('a', 'Utf8View'), null);
 ----
 a
+
+# Test with dictionary-encoded strings
+# This tests the fix for: "Dictionary(UInt32, Utf8) and Utf8 is not comparable"
+statement ok
+CREATE TABLE dict_test_base(
+  col1 TEXT,
+  col2 TEXT
+) as VALUES
+  ('foo', 'bar'),
+  ('bar', 'bar'),
+  ('baz', 'bar')
+;
+
+# Dictionary cast with string literal
+query T rowsort
+SELECT NULLIF(arrow_cast(col1, 'Dictionary(Int32, Utf8)'), 'bar') FROM dict_test_base;
+----
+NULL
+baz
+foo
+
+# String with dictionary cast
+query T rowsort
+SELECT NULLIF(col2, arrow_cast(col1, 'Dictionary(Int32, Utf8)')) FROM dict_test_base;
+----
+NULL
+bar
+bar
+
+# Both as dictionaries
+query T rowsort
+SELECT NULLIF(arrow_cast(col1, 'Dictionary(Int32, Utf8)'), arrow_cast('bar', 'Dictionary(Int32, Utf8)')) FROM dict_test_base;
+----
+NULL
+baz
+foo