Merge branch 'clear-up-background-type-change-docs' into 'master'
Add docs for the cleanup step of a background column type change See merge request gitlab-org/gitlab-ce!20020
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commit
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@ -252,6 +252,53 @@ Keep in mind that the relation passed to
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`change_column_type_using_background_migration` _must_ include `EachBatch`,
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otherwise it will raise a `TypeError`.
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This migration then needs to be followed in a separate release (_not_ a patch
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release) by a cleanup migration, which should steal from the queue and handle
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any remaining rows. For example:
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```ruby
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class MigrateRemainingIssuesClosedAt < ActiveRecord::Migration
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include Gitlab::Database::MigrationHelpers
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DOWNTIME = false
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disable_ddl_transaction!
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class Issue < ActiveRecord::Base
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self.table_name = 'issues'
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include EachBatch
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end
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def up
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Gitlab::BackgroundMigration.steal('CopyColumn')
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Gitlab::BackgroundMigration.steal('CleanupConcurrentTypeChange')
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migrate_remaining_rows if migrate_column_type?
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end
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def down
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# Previous migrations already revert the changes made here.
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end
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def migrate_remaining_rows
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Issue.where('closed_at_for_type_change IS NULL AND closed_at IS NOT NULL').each_batch do |batch|
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batch.update_all('closed_at_for_type_change = closed_at')
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end
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cleanup_concurrent_column_type_change(:issues, :closed_at)
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end
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def migrate_column_type?
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# Some environments may have already executed the previous version of this
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# migration, thus we don't need to migrate those environments again.
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column_for('issues', 'closed_at').type == :datetime # rubocop:disable Migration/Datetime
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end
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end
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```
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For more information, see [the documentation on cleaning up background
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migrations](background_migrations.md#cleaning-up).
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## Adding Indexes
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Adding indexes is an expensive process that blocks INSERT and UPDATE queries for
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