CHANGELOG & improvements to #16649

* Require either FIRST or LAST qualifier for "NULLS ..."
* Require whitespace before "NULLS ..."

activerecord/CHANGELOG.md

@@ -1,3 +1,10 @@
+*   Fixed a regression where whitespaces were stripped from DISTINCT queries in
+    PostgreSQL.
+
+    *Agis Anastasopoulos*
+
+    Fixes #16623.
+
 *   Fix has_many :through relation merging failing when dynamic conditions are
     passed as a lambda with an arity of one.
 

activerecord/lib/active_record/connection_adapters/postgresql/schema_statements.rb

@@ -549,8 +549,8 @@ def columns_for_distinct(columns, orders) #:nodoc:
               # Convert Arel node to string
               s = s.to_sql unless s.is_a?(String)
               # Remove any ASC/DESC modifiers
-              s.gsub(/\s+(?:ASC|DESC)\s*/i, '')
-               .gsub(/\s*NULLS\s+(?:FIRST|LAST)?\s*/i, '')
+              s.gsub(/\s+(?:ASC|DESC)\b/i, '')
+               .gsub(/\s+NULLS\s+(?:FIRST|LAST)\b/i, '')
             }.reject(&:blank?).map.with_index { |column, i| "#{column} AS alias_#{i}" }
 
           [super, *order_columns].join(', ')