diff --git a/activerecord/CHANGELOG.md b/activerecord/CHANGELOG.md
index b3ae1c156a697..6924576991dc2 100644
--- a/activerecord/CHANGELOG.md
+++ b/activerecord/CHANGELOG.md
@@ -1,3 +1,10 @@
+*   Fixed a regression where whitespaces were stripped from DISTINCT queries in
+    PostgreSQL.
+
+    *Agis Anastasopoulos*
+
+    Fixes #16623.
+
 *   Fix regression on after_commit that didnt fire when having nested transactions.
 
     Fixes #16425
diff --git a/activerecord/lib/active_record/connection_adapters/postgresql/schema_statements.rb b/activerecord/lib/active_record/connection_adapters/postgresql/schema_statements.rb
index c2017b0af2ade..7f5a567c29746 100644
--- a/activerecord/lib/active_record/connection_adapters/postgresql/schema_statements.rb
+++ b/activerecord/lib/active_record/connection_adapters/postgresql/schema_statements.rb
@@ -493,8 +493,8 @@ 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(', ')