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