use PostgreSQL's bulk_alter_table implementation

Running this migration on mysql at current master fails
because `add_references_for_alter` is missing.

```
change_table :users, bulk: true do |t|
  t.references :article
end
```

This is also true for postgresql adapter,
but its `bulk_alter_table` implementation can fallback in such case.
postgresql's implementation is desirable to prevent unknown failure like this.

3 files changed, 26 insertions, 41 deletions

diff --git a/activerecord/lib/active_record/connection_adapters/abstract/schema_statements.rb b/activerecord/lib/active_record/connection_adapters/abstract/schema_statements.rb
index 4840307094..8ba9943f75 100644
--- a/activerecord/lib/active_record/connection_adapters/abstract/schema_statements.rb
+++ b/activerecord/lib/active_record/connection_adapters/abstract/schema_statements.rb
@@ -1379,6 +1379,31 @@ module ActiveRecord
           options.is_a?(Hash) && options.key?(:name) && options.except(:name, :algorithm).empty?
         end
 
+        def bulk_change_table(table_name, operations)
+          sql_fragments = []
+          non_combinable_operations = []
+
+          operations.each do |command, args|
+            table, arguments = args.shift, args
+            method = :"#{command}_for_alter"
+
+            if respond_to?(method, true)
+              sqls, procs = Array(send(method, table, *arguments)).partition { |v| v.is_a?(String) }
+              sql_fragments << sqls
+              non_combinable_operations.concat(procs)
+            else
+              execute "ALTER TABLE #{quote_table_name(table_name)} #{sql_fragments.join(", ")}" unless sql_fragments.empty?
+              non_combinable_operations.each(&:call)
+              sql_fragments = []
+              non_combinable_operations = []
+              send(command, table, *arguments)
+            end
+          end
+
+          execute "ALTER TABLE #{quote_table_name(table_name)} #{sql_fragments.join(", ")}" unless sql_fragments.empty?
+          non_combinable_operations.each(&:call)
+        end
+
         def add_column_for_alter(table_name, column_name, type, options = {})
           td = create_table_definition(table_name)
           cd = td.new_column_definition(column_name, type, options)
diff --git a/activerecord/lib/active_record/connection_adapters/abstract_mysql_adapter.rb b/activerecord/lib/active_record/connection_adapters/abstract_mysql_adapter.rb
index ca8bbc14da..2a2b234ef8 100644
--- a/activerecord/lib/active_record/connection_adapters/abstract_mysql_adapter.rb
+++ b/activerecord/lib/active_record/connection_adapters/abstract_mysql_adapter.rb
@@ -63,7 +63,7 @@ module ActiveRecord
         /mariadb/i.match?(full_version)
       end
 
-      def supports_bulk_alter? #:nodoc:
+      def supports_bulk_alter?
         true
       end
 
@@ -285,21 +285,6 @@ module ActiveRecord
         SQL
       end
 
-      def bulk_change_table(table_name, operations) #:nodoc:
-        sqls = operations.flat_map do |command, args|
-          table, arguments = args.shift, args
-          method = :"#{command}_for_alter"
-
-          if respond_to?(method, true)
-            send(method, table, *arguments)
-          else
-            raise "Unknown method called : #{method}(#{arguments.inspect})"
-          end
-        end.join(", ")
-
-        execute("ALTER TABLE #{quote_table_name(table_name)} #{sqls}")
-      end
-
       def change_table_comment(table_name, comment) #:nodoc:
         comment = "" if comment.nil?
         execute("ALTER TABLE #{quote_table_name(table_name)} COMMENT #{quote(comment)}")
diff --git a/activerecord/lib/active_record/connection_adapters/postgresql/schema_statements.rb b/activerecord/lib/active_record/connection_adapters/postgresql/schema_statements.rb
index 2a641cbe53..ec8af73d07 100644
--- a/activerecord/lib/active_record/connection_adapters/postgresql/schema_statements.rb
+++ b/activerecord/lib/active_record/connection_adapters/postgresql/schema_statements.rb
@@ -368,31 +368,6 @@ module ActiveRecord
           SQL
         end
 
-        def bulk_change_table(table_name, operations)
-          sql_fragments = []
-          non_combinable_operations = []
-
-          operations.each do |command, args|
-            table, arguments = args.shift, args
-            method = :"#{command}_for_alter"
-
-            if respond_to?(method, true)
-              sqls, procs = Array(send(method, table, *arguments)).partition { |v| v.is_a?(String) }
-              sql_fragments << sqls
-              non_combinable_operations.concat(procs)
-            else
-              execute "ALTER TABLE #{quote_table_name(table_name)} #{sql_fragments.join(", ")}" unless sql_fragments.empty?
-              non_combinable_operations.each(&:call)
-              sql_fragments = []
-              non_combinable_operations = []
-              send(command, table, *arguments)
-            end
-          end
-
-          execute "ALTER TABLE #{quote_table_name(table_name)} #{sql_fragments.join(", ")}" unless sql_fragments.empty?
-          non_combinable_operations.each(&:call)
-        end
-
         # Renames a table.
         # Also renames a table's primary key sequence if the sequence name exists and
         # matches the Active Record default.