Merge pull request rails#46592 from pirj/dont-use-lower-for-case-insensitive-queries-on-citext

Don't use lower() for citext columns

Author: yahonda

activerecord/CHANGELOG.md

@@ -1,3 +1,12 @@
+*   Stop using `LOWER()` for case-insensitive queries on `citext` columns
+
+    Previously, `LOWER()` was added for e.g. uniqueness validations with
+    `case_sensitive: false`.
+    It wasn't mentioned in the documentation that the index without `LOWER()`
+    wouldn't be used in this case.
+
+    *Phil Pirozhkov*
+
 *   Extract `#sync_timezone_changes` method in AbstractMysqlAdapter to enable subclasses
     to sync database timezone changes without overriding `#raw_execute`.
 

activerecord/lib/active_record/connection_adapters/postgresql_adapter.rb

@@ -1031,7 +1031,10 @@ def build_statement_pool
         end
 
         def can_perform_case_insensitive_comparison_for?(column)
-          @case_insensitive_cache ||= {}
+          # NOTE: citext is an exception. It is possible to perform a
+          #       case-insensitive comparison using `LOWER()`, but it is
+          #       unnecessary, as `citext` is case-insensitive by definition.
+          @case_insensitive_cache ||= { "citext" => false }
           @case_insensitive_cache.fetch(column.sql_type) do
             @case_insensitive_cache[column.sql_type] = begin
               sql = <<~SQL

activerecord/test/cases/adapters/postgresql/citext_test.rb

@@ -71,6 +71,12 @@ def test_select_case_insensitive
     assert_equal "Cased Text", x.cival
   end
 
+  def test_case_insensitiveness
+    attr = Citext.arel_table[:cival]
+    comparison = @connection.case_insensitive_comparison(attr, nil)
+    assert_no_match(/lower/i, comparison.to_sql)
+  end
+
   def test_schema_dump_with_shorthand
     output = dump_table_schema("citexts")
     assert_match %r[t\.citext "cival"], output