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, 10 insertions, 1 deletions

diff --git a/activerecord/lib/active_record/connection_adapters/mysql/type_metadata.rb b/activerecord/lib/active_record/connection_adapters/mysql/type_metadata.rb
index 9167593064..a7232fa249 100644
--- a/activerecord/lib/active_record/connection_adapters/mysql/type_metadata.rb
+++ b/activerecord/lib/active_record/connection_adapters/mysql/type_metadata.rb
@@ -6,9 +6,11 @@ module ActiveRecord
       class TypeMetadata < DelegateClass(SqlTypeMetadata) # :nodoc:
         undef to_yaml if method_defined?(:to_yaml)
 
+        include Deduplicable
+
         attr_reader :extra
 
-        def initialize(type_metadata, extra: "")
+        def initialize(type_metadata, extra: nil)
           super(type_metadata)
           @extra = extra
         end
@@ -25,6 +27,13 @@ module ActiveRecord
             __getobj__.hash ^
             extra.hash
         end
+
+        private
+          def deduplicated
+            __setobj__(__getobj__.deduplicate)
+            @extra = -extra if extra
+            super
+          end
       end
     end
   end