Question

I’m fetching a record by the code just like these

@community = Community.find_by_community_name(params[:community_name])
@user = User.find_by_username(params[:username])

I want to make it faster loading, so I’m thinking of adding index to them just like this.
If I do rake db:migrate, does it reindex to the existing records also?
Or just the records that will be created from now on?
Do it improve the speed of loading by adding index just like this?

class AddIndexToCommunity < ActiveRecord::Migration
  def self.up
    add_index :communities, [:community_name, :title, :body], :name=>:communities_idx
  end

  def self.down
    remove_index :communities, [:community_name, :title, :body], :name=>:communities_idx
  end
end



class AddIndexToUser < ActiveRecord::Migration
  def self.up
    add_index :users, [:username, :nickname, :body], :name=>:users_idx
  end

  def self.down
    remove_index :users, [:username, :nickname, :body], :name=>:users_idx
  end
end

Answer 1

rake db:migrate will perform database migrations and apply indeces immediately.You should apply indeces only to columns which you use in searching. Remember that indeces add time penalty on insert and update operations. If you load records on by their names, add indeces only to names:

class AddIndexToCommunity < ActiveRecord::Migration
  def self.up
    add_index :communities, :community_name
  end

  def self.down
    remove_index :communities, :community_name
  end
end



class AddIndexToUser < ActiveRecord::Migration
  def self.up
    add_index :users, :username
  end

  def self.down
    remove_index :users, :username
  end
end