Remove indexes for FK

db/migrate/20181030154446_add_missing_indexes_for_foreign_keys.rb

@@ -33,6 +33,31 @@ class AddMissingIndexesForForeignKeys < ActiveRecord::Migration
   end
 
   def down
-    # no-op
+    # MySQL requires index for FK,
+    # thus removal of indexes does fail
+    return if Gitlab::Database.mysql?
+
+    remove_concurrent_index(:application_settings, :usage_stats_set_by_user_id)
+    remove_concurrent_index(:ci_pipeline_schedules, :owner_id)
+    remove_concurrent_index(:ci_trigger_requests, :trigger_id)
+    remove_concurrent_index(:ci_triggers, :owner_id)
+    remove_concurrent_index(:clusters_applications_helm, :cluster_id, unique: true)
+    remove_concurrent_index(:clusters_applications_ingress, :cluster_id, unique: true)
+    remove_concurrent_index(:clusters_applications_jupyter, :cluster_id, unique: true)
+    remove_concurrent_index(:clusters_applications_jupyter, :oauth_application_id)
+    remove_concurrent_index(:clusters_applications_prometheus, :cluster_id, unique: true)
+    remove_concurrent_index(:fork_network_members, :forked_from_project_id)
+    remove_concurrent_index(:internal_ids, :namespace_id)
+    remove_concurrent_index(:internal_ids, :project_id)
+    remove_concurrent_index(:issues, :closed_by_id)
+    remove_concurrent_index(:label_priorities, :label_id)
+    remove_concurrent_index(:merge_request_metrics, :merged_by_id)
+    remove_concurrent_index(:merge_request_metrics, :latest_closed_by_id)
+    remove_concurrent_index(:oauth_openid_requests, :access_grant_id)
+    remove_concurrent_index(:project_deploy_tokens, :deploy_token_id)
+    remove_concurrent_index(:protected_tag_create_access_levels, :group_id)
+    remove_concurrent_index(:subscriptions, :project_id)
+    remove_concurrent_index(:user_statuses, :user_id)
+    remove_concurrent_index(:users, :accepted_term_id)
   end
 end