Migration

  • Rails runs each migration inside a transaction.
  • If the migration fails, the transaction is rolled back.

Example: time taken removing the index:#

  • User table has 1 Million rows:

  • It was pretty fast. 3 Milliseconds for 1 million rows.

  • Migration

class RemoveIndexFromUser < ActiveRecord::Migration[6.0]
def up
remove_index :users, name: :index_users_on_id
end
end
  • Output on console
== 20201229223838 RemoveIndexFromUser: migrating ===============================
-- remove_index(:users, {:name=>:index_users_on_id})
-> 0.0367s
== 20201229223838 RemoveIndexFromUser: migrated (0.0368s) ======================

Example: time taken adding the index:#

  • User table has 1 million rows:

  • It's slow. 13 seconds for 1 million rows.

  • Imagine what happens when you have 10/100 Million rows?

  • This will lock that table for that long.

  • Migration

class AddIndexOnUser < ActiveRecord::Migration[6.0]
def up
add_index :users, :id
end
end
  • Output on console
== 20201229223838 AddIndexToUser: migrating ===============================
-- add_index(:users, :id)
-> 13.1013s
== 20201229223838 AddIndexToUser: migrated (13.1014s) =====================

Problem ?#

warning

It will basically cause a downtime for 13 seconds

Solution#

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