Postgres, fix test only check for insensitive comparison capability once

8) Failure:
ActiveRecord::ConnectionAdapters::PostgreSQLAdapterTest#test_only_check_for_insensitive_comparison_capability_once [test/cases/adapters/postgresql/postgresql_adapter_test.rb:539]:
1 instead of 0 queries were executed.
Queries:
SELECT exists(
  SELECT * FROM pg_proc
  WHERE proname = 'lower'
    AND proargtypes = ARRAY['example_type'::regtype]::oidvector
) OR exists(
  SELECT * FROM pg_proc
  INNER JOIN pg_cast
    ON ARRAY[casttarget]::oidvector = proargtypes
  WHERE proname = 'lower'
    AND castsource = 'example_type'::regtype
)
.
Expected: 0
  Actual: 1

Author: JesseChavez

lib/arjdbc/postgresql/adapter.rb

@@ -667,22 +667,27 @@ def arel_visitor
 
     # Pulled from ActiveRecord's Postgres adapter and modified to use execute
     def can_perform_case_insensitive_comparison_for?(column)
-      @case_insensitive_cache ||= {}
-      @case_insensitive_cache[column.sql_type] ||= begin
-        sql = <<~SQL
-          SELECT exists(
-            SELECT * FROM pg_proc
-            WHERE proname = 'lower'
-              AND proargtypes = ARRAY[#{quote column.sql_type}::regtype]::oidvector
-          ) OR exists(
-            SELECT * FROM pg_proc
-            INNER JOIN pg_cast
-              ON ARRAY[casttarget]::oidvector = proargtypes
-            WHERE proname = 'lower'
-              AND castsource = #{quote column.sql_type}::regtype
-          )
-        SQL
-        select_value(sql, 'SCHEMA')
+      # 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
+            SELECT exists(
+              SELECT * FROM pg_proc
+              WHERE proname = 'lower'
+                AND proargtypes = ARRAY[#{quote column.sql_type}::regtype]::oidvector
+            ) OR exists(
+              SELECT * FROM pg_proc
+              INNER JOIN pg_cast
+                ON ARRAY[casttarget]::oidvector = proargtypes
+              WHERE proname = 'lower'
+                AND castsource = #{quote column.sql_type}::regtype
+            )
+          SQL
+          select_value(sql, 'SCHEMA')
+        end
       end
     end