1 files changed, 43 insertions, 0 deletions
diff --git a/db/post_migrate/20180906051323_remove_orphaned_label_links.rb b/db/post_migrate/20180906051323_remove_orphaned_label_links.rb
new file mode 100644
index 00000000000..b56b74f483e
--- /dev/null
+++ b/db/post_migrate/20180906051323_remove_orphaned_label_links.rb
@@ -0,0 +1,43 @@
+# frozen_string_literal: true
+
+class RemoveOrphanedLabelLinks < ActiveRecord::Migration
+  include Gitlab::Database::MigrationHelpers
+
+  DOWNTIME = false
+
+  disable_ddl_transaction!
+
+  class LabelLinks < ActiveRecord::Base
+    self.table_name = 'label_links'
+    include EachBatch
+
+    def self.orphaned
+      where('NOT EXISTS ( SELECT 1 FROM labels WHERE labels.id = label_links.label_id )')
+    end
+  end
+
+  def up
+    # Some of these queries can take up to 10 seconds to run on GitLab.com,
+    # which is pretty close to our 15 second statement timeout. To ensure a
+    # smooth deployment procedure we disable the statement timeouts for this
+    # migration, just in case.
+    disable_statement_timeout do
+      # On GitLab.com there are over 2,000,000 orphaned label links. On
+      # staging, removing 100,000 rows generated a max replication lag of 6.7
+      # MB. In total, removing all these rows will only generate about 136 MB
+      # of data, so it should be safe to do this.
+      LabelLinks.orphaned.each_batch(of: 100_000) do |batch|
+        batch.delete_all
+      end
+    end
+
+    add_concurrent_foreign_key(:label_links, :labels, column: :label_id, on_delete: :cascade)
+  end
+
+  def down
+    # There is no way to restore orphaned label links.
+    if foreign_key_exists?(:label_links, column: :label_id)
+      remove_foreign_key(:label_links, column: :label_id)
+    end
+  end
+end