# Swapping Tables Sometimes you need to replace one table with another. For example, when migrating data in a very large table it's often better to create a copy of the table and insert & migrate the data into this new table in the background. Let's say you want to swap the table "events" with "events_for_migration". In this case you need to follow 3 steps: 1. Rename "events" to "events_temporary" 1. Rename "events_for_migration" to "events" 1. Rename "events_temporary" to "events_for_migration" Rails allows you to do this using the `rename_table` method: ```ruby rename_table :events, :events_temporary rename_table :events_for_migration, :events rename_table :events_temporary, :events_for_migration ``` This does not require any downtime as long as the 3 `rename_table` calls are executed in the _same_ database transaction. Rails by default uses database transactions for migrations, but if it doesn't you'll need to start one manually: ```ruby Event.transaction do rename_table :events, :events_temporary rename_table :events_for_migration, :events rename_table :events_temporary, :events_for_migration end ``` Once swapped you _have to_ reset the primary key of the new table. For PostgreSQL you can use the `reset_pk_sequence!` method like so: ```ruby reset_pk_sequence!('events') ``` For MySQL however you need to do run the following: ```ruby amount = Event.pluck('COALESCE(MAX(id), 1)').first execute "ALTER TABLE events AUTO_INCREMENT = #{amount}" ``` Failure to reset the primary keys will result in newly created rows starting with an ID value of 1. Depending on the existing data this can then lead to duplicate key constraints from popping up, preventing users from creating new data.