A Rails plugin that adds methods to ActiveRecord::Migration to allow columns and indexes to be added to and removed from large tables with millions of rows in MySQL, without leaving processes seemingly stalled in state "copy to tmp table".

For each of the standard transformations that operate on columns or indexes, this plugin adds a "using_tmp_table" version. These methods create a temporary table with the same structure as the table to be altered, applies the transformation to the temp table, copies data from the source table to the temp table and then replaces the source table with the temporary one.

While it does try to ensure that data is consistent at the end of the entire process by locking tables and looking for rows created or modified during copying, this is NOT TRANSACTION SAFE as it (a) relies on timestamp columns and (b) doesn't handle rows that have been deleted from the source table after being copied to the temporary table.

source 'https://rubygems.org' do
  ...
  gem 'mysql_big_table_migration'
  ...
end

class AddIndexOnSomeColumnToSomeTable < ActiveRecord::Migration
  def self.up
    add_index_using_tmp_table :some_table, :some_column
  end

  def self.down
    remove_index_using_tmp_table :some_table, :some_column
  end
end

class MungeOtherTable < ActiveRecord::Migration
  def change
    with_tmp_table :some_table do |t|
      remove_column t, :boring_data
      remove_index t, name: :index_some_table_on_useful_data_and_boring_data
      add_column t, :other_id, :integer
      change_column t, :comment, :string, limit: 140
      add_index t, :other_id, unique: true, name: :idx_other_id
    end
  end
end

Copyright (c) 2010 Mark Woods, released under the MIT license

You will need to bundle install dependencies for the project, as well as create a test database. To install dependencies run bundle install. To create the database run the following in a MySQL prompt as an admin user:

CREATE DATABASE mysql_big_table_migration_test;
GRANT ALL PRIVILEGES ON mysql_big_table_migration_test.* TO 'travis'@'localhost';

After bundling dependencies, and setting up the test database, run the tests with rake test