Updated migration styleguide for new helpers

doc/development/migration_style_guide.md

@@ -8,7 +8,10 @@ In addition, having to take a server offline for a an upgrade small or big is
 a big burden for most organizations. For this reason it is important that your
 migrations are written carefully, can be applied online and adhere to the style guide below.
 
-It's advised to have offline migrations only in major GitLab releases.
+Migrations should not require GitLab installations to be taken offline unless
+_absolutely_ necessary. If a migration requires downtime this should be
+clearly mentioned during the review process as well as being documented in the
+monthly release post.
 
 When writing your migrations, also consider that databases might have stale data
 or inconsistencies and guard for that. Try to make as little assumptions as possible
@@ -58,6 +61,45 @@ remove_index :namespaces, column: :name if index_exists?(:namespaces, :name)
 
 If you need to add an unique index please keep in mind there is possibility of existing duplicates. If it is possible write a separate migration for handling this situation. It can be just removing or removing with overwriting all references to these duplicates depend on situation.
 
+When adding an index make sure to use the method `add_concurrent_index` instead
+of the regular `add_index` method. The `add_concurrent_index` method
+automatically creates concurrent indexes when using PostgreSQL, removing the
+need for downtime. To use this method you must disable transactions by calling
+the method `disable_ddl_transaction!` in the body of your migration class like
+so:
+
+```
+class MyMigration < ActiveRecord::Migration
+  disable_ddl_transaction!
+
+  def change
+
+  end
+end
+```
+
+## Adding Columns With Default Values
+
+When adding columns with default values you should use the method
+`add_column_with_default`. This method ensures the table is updated without
+requiring downtime. This method is not reversible so you must manually define
+the `up` and `down` methods in your migration class.
+
+For example, to add the column `foo` to the `projects` table with a default
+value of `10` you'd write the following:
+
+```
+class MyMigration < ActiveRecord::Migration
+  def up
+    add_column_with_default(:projects, :foo, :integer, 10)
+  end
+
+  def down
+    remove_column(:projects, :foo)
+  end
+end
+```
+
 ## Testing
 
 Make sure that your migration works with MySQL and PostgreSQL with data. An empty database does not guarantee that your migration is correct.