Remove inconsistent index

- and restore it exactly upon down

changelogs/unreleased/296756-cablett-remove-index.yml

+---
+title: Remove inconsistent index in backup_labels table
+merge_request: 53189
+author:
+type: other

db/migrate/20210203002331_drop_backup_label_index.rb

+# frozen_string_literal: true
+
+class DropBackupLabelIndex < ActiveRecord::Migration[6.0]
+  include Gitlab::Database::MigrationHelpers
+
+  DOWNTIME = false
+
+  disable_ddl_transaction!
+
+  INDEX_NAME = 'backup_labels_project_id_title_idx'
+
+  def up
+    remove_concurrent_index_by_name(:backup_labels, name: INDEX_NAME)
+  end
+
+  def down
+    add_concurrent_index :backup_labels, [:project_id, :title], name: INDEX_NAME, unique: true, where: 'group_id = NULL::integer'
+  end
+end

db/schema_migrations/20210203002331

+b8a19b2ebf648a1b79a488a7a2847f6a392e594464c1ba0ae96f274f583559e3
\ No newline at end of file

db/structure.sql

@@ -21023,8 +21023,6 @@ CREATE INDEX backup_labels_group_id_title_idx ON backup_labels USING btree (grou
 
 CREATE INDEX backup_labels_project_id_idx ON backup_labels USING btree (project_id);
 
-CREATE UNIQUE INDEX backup_labels_project_id_title_idx ON backup_labels USING btree (project_id, title) WHERE (group_id = NULL::integer);
-
 CREATE INDEX backup_labels_template_idx ON backup_labels USING btree (template) WHERE template;
 
 CREATE INDEX backup_labels_title_idx ON backup_labels USING btree (title);