Deduplicate various Active Record schema cache structures

Real world database schemas contain a lot of duplicated data.
Some column names like `id`, `created_at` etc can easily be repeated
hundreds of times. Same for SqlTypeMetada, most database will contain
only a limited number of possible combinations.

This result in a lot of wasted memory.

The idea here is to make these data sctructures immutable, use a registry
to substitute similar instances with pre-existing ones.

2 files changed, 7 insertions, 4 deletions

diff --git a/activerecord/test/cases/base_test.rb b/activerecord/test/cases/base_test.rb
index ddafa468ed..1c83100a75 100644
--- a/activerecord/test/cases/base_test.rb
+++ b/activerecord/test/cases/base_test.rb
@@ -1141,11 +1141,14 @@ class BasicsTest < ActiveRecord::TestCase
   def test_clear_cache!
     # preheat cache
     c1 = Post.connection.schema_cache.columns("posts")
+    assert_not_equal 0, Post.connection.schema_cache.size
+
     ActiveRecord::Base.clear_cache!
+    assert_equal 0, Post.connection.schema_cache.size
+
     c2 = Post.connection.schema_cache.columns("posts")
-    c1.each_with_index do |v, i|
-      assert_not_same v, c2[i]
-    end
+    assert_not_equal 0, Post.connection.schema_cache.size
+
     assert_equal c1, c2
   end
 
diff --git a/activerecord/test/cases/json_serialization_test.rb b/activerecord/test/cases/json_serialization_test.rb
index 82cf281cff..8f9b1e2ec1 100644
--- a/activerecord/test/cases/json_serialization_test.rb
+++ b/activerecord/test/cases/json_serialization_test.rb
@@ -24,7 +24,7 @@ class JsonSerializationTest < ActiveRecord::TestCase
   include JsonSerializationHelpers
 
   class NamespacedContact < Contact
-    column :name, :string
+    column :name, "string"
   end
 
   def setup