52 lines
1.7 KiB
Markdown
52 lines
1.7 KiB
Markdown
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---
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stage: Data Stores
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group: Database
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info: To determine the technical writer assigned to the Stage/Group associated with this page, see https://about.gitlab.com/handbook/engineering/ux/technical-writing/#assignments
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---
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# 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 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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Failure to reset the primary keys results 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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