add trigram database index on tags

db/migrate/20190206193120_add_index_to_tags.rb

+# frozen_string_literal: true
+
+class AddIndexToTags < ActiveRecord::Migration[5.0]
+  include Gitlab::Database::MigrationHelpers
+
+  DOWNTIME = false
+  INDEX_NAME = 'index_tags_on_name_trigram'
+
+  disable_ddl_transaction!
+
+  def up
+    add_concurrent_index :tags, :name, name: INDEX_NAME, using: :gin, opclasses: { name: :gin_trgm_ops }
+  end
+
+  def down
+    remove_concurrent_index_by_name(:tags, INDEX_NAME)
+  end
+end

db/schema.rb

@@ -2053,6 +2053,7 @@ ActiveRecord::Schema.define(version: 20190220150130) do
     t.string "name"
     t.integer "taggings_count", default: 0
     t.index ["name"], name: "index_tags_on_name", unique: true, using: :btree
+    t.index ["name"], name: "index_tags_on_name_trigram", using: :gin, opclasses: {"name"=>"gin_trgm_ops"}
   end
 
   create_table "term_agreements", force: :cascade do |t|