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.

1 files changed, 29 insertions, 0 deletions

diff --git a/activerecord/lib/active_record/connection_adapters/deduplicable.rb b/activerecord/lib/active_record/connection_adapters/deduplicable.rb
new file mode 100644
index 0000000000..fb2fd60bbc
--- /dev/null
+++ b/activerecord/lib/active_record/connection_adapters/deduplicable.rb
@@ -0,0 +1,29 @@
+# frozen_string_literal: true
+
+module ActiveRecord
+  module ConnectionAdapters # :nodoc:
+    module Deduplicable
+      extend ActiveSupport::Concern
+
+      module ClassMethods
+        def registry
+          @registry ||= {}
+        end
+
+        def new(*)
+          super.deduplicate
+        end
+      end
+
+      def deduplicate
+        self.class.registry[self] ||= deduplicated
+      end
+      alias :-@ :deduplicate
+
+      private
+        def deduplicated
+          freeze
+        end
+    end
+  end
+end