Commit 9a65af63 authored by Sean McGivern's avatar Sean McGivern

Add docs for the cleanup step of a background column type change

A background column type change needs a cleanup step, like any background
migration. We should call that out explicitly in the documentation.
parent c0f0ccf2
......@@ -252,6 +252,53 @@ Keep in mind that the relation passed to
`change_column_type_using_background_migration` _must_ include `EachBatch`,
otherwise it will raise a `TypeError`.
This migration then needs to be followed in a separate release (_not_ a patch
release) by a cleanup migration, which should steal from the queue and handle
any remaining rows. For example:
```ruby
class MigrateRemainingIssuesClosedAt < ActiveRecord::Migration
include Gitlab::Database::MigrationHelpers
DOWNTIME = false
disable_ddl_transaction!
class Issue < ActiveRecord::Base
self.table_name = 'issues'
include EachBatch
end
def up
Gitlab::BackgroundMigration.steal('CopyColumn')
Gitlab::BackgroundMigration.steal('CleanupConcurrentTypeChange')
migrate_remaining_rows if migrate_column_type?
end
def down
# Previous migrations already revert the changes made here.
end
def migrate_remaining_rows
Issue.where('closed_at_for_type_change IS NULL AND closed_at IS NOT NULL').each_batch do |batch|
batch.update_all('closed_at_for_type_change = closed_at')
end
cleanup_concurrent_column_type_change(:issues, :closed_at)
end
def migrate_column_type?
# Some environments may have already executed the previous version of this
# migration, thus we don't need to migrate those environments again.
column_for('issues', 'closed_at').type == :datetime # rubocop:disable Migration/Datetime
end
end
```
For more information, see [the documentation on cleaning up background
migrations](background_migrations.md#cleaning-up).
## Adding Indexes
Adding indexes is an expensive process that blocks INSERT and UPDATE queries for
......
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