Add two partial indexes

- Non-unique

changelogs/unreleased/30390-cablett-duplicate-labels-nonunique-index.yml

+---
+title: Add nonunique indexes to Labels
+merge_request: 21230
+author:
+type: fixed

db/migrate/20191204093410_add_label_project_group_partial_indexes.rb

+# frozen_string_literal: true
+
+# See http://doc.gitlab.com/ce/development/migration_style_guide.html
+# for more information on how to write migrations for GitLab.
+
+class AddLabelProjectGroupPartialIndexes < ActiveRecord::Migration[5.2]
+  include Gitlab::Database::MigrationHelpers
+
+  DOWNTIME = false
+
+  disable_ddl_transaction!
+
+  PROJECT_AND_TITLE = [:project_id, :title]
+  GROUP_AND_TITLE = [:group_id, :title]
+
+  def up
+    add_concurrent_index :labels, PROJECT_AND_TITLE, unique: false, where: "labels.group_id = null"
+    add_concurrent_index :labels, GROUP_AND_TITLE, unique: false, where: "labels.project_id = null"
+  end
+
+  def down
+    remove_concurrent_index :labels, PROJECT_AND_TITLE
+    remove_concurrent_index :labels, GROUP_AND_TITLE
+  end
+end

db/schema.rb

@@ -10,7 +10,7 @@
 #
 # It's strongly recommended that you check this file into your version control system.
 
-ActiveRecord::Schema.define(version: 2019_12_04_070713) do
+ActiveRecord::Schema.define(version: 2019_12_04_093410) do
 
   # These are extensions that must be enabled in order to support this database
   enable_extension "pg_trgm"
@@ -2228,6 +2228,8 @@ ActiveRecord::Schema.define(version: 2019_12_04_070713) do
     t.integer "group_id"
     t.integer "cached_markdown_version"
     t.index ["group_id", "project_id", "title"], name: "index_labels_on_group_id_and_project_id_and_title", unique: true
+    t.index ["group_id", "title"], name: "index_labels_on_group_id_and_title", where: "(project_id = NULL::integer)"
+    t.index ["project_id", "title"], name: "index_labels_on_project_id_and_title", where: "(group_id = NULL::integer)"
     t.index ["project_id"], name: "index_labels_on_project_id"
     t.index ["template"], name: "index_labels_on_template", where: "template"
     t.index ["title"], name: "index_labels_on_title"