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, 21 insertions, 0 deletions

diff --git a/activerecord/lib/active_record/connection_adapters/postgresql/column.rb b/activerecord/lib/active_record/connection_adapters/postgresql/column.rb
index ec25bb1e19..b4ea6c42d3 100644
--- a/activerecord/lib/active_record/connection_adapters/postgresql/column.rb
+++ b/activerecord/lib/active_record/connection_adapters/postgresql/column.rb
@@ -23,6 +23,19 @@ module ActiveRecord
         def sql_type
           super.sub(/\[\]\z/, "")
         end
+
+        def ==(other)
+          other.is_a?(Column) &&
+            super &&
+            serial? == other.serial?
+        end
+        alias :eql? :==
+
+        def hash
+          Column.hash ^
+            super.hash ^
+            serial?.hash
+        end
       end
     end
     PostgreSQLColumn = PostgreSQL::Column # :nodoc:
diff --git a/activerecord/lib/active_record/connection_adapters/postgresql/type_metadata.rb b/activerecord/lib/active_record/connection_adapters/postgresql/type_metadata.rb
index 8bdec623af..b7f6479357 100644
--- a/activerecord/lib/active_record/connection_adapters/postgresql/type_metadata.rb
+++ b/activerecord/lib/active_record/connection_adapters/postgresql/type_metadata.rb
@@ -7,6 +7,8 @@ module ActiveRecord
       class TypeMetadata < DelegateClass(SqlTypeMetadata)
         undef to_yaml if method_defined?(:to_yaml)
 
+        include Deduplicable
+
         attr_reader :oid, :fmod
 
         def initialize(type_metadata, oid: nil, fmod: nil)
@@ -29,6 +31,12 @@ module ActiveRecord
             oid.hash ^
             fmod.hash
         end
+
+        private
+          def deduplicated
+            __setobj__(__getobj__.deduplicate)
+            super
+          end
       end
     end
     PostgreSQLTypeMetadata = PostgreSQL::TypeMetadata