Fix concurrent index problem

db/migrate/20180831164908_add_identifier_to_prometheus_metric.1.rb

@@ -7,6 +7,5 @@ class AddIdentifierToPrometheusMetric < ActiveRecord::Migration
 
   def change
     add_column :prometheus_metrics, :identifier, :string
-    add_index :prometheus_metrics, :identifier, unique: true
   end
 end

db/migrate/20180831164909_add_index_for_identifier_to_prometheus_metric.rb

@@ -0,0 +1,17 @@
+# frozen_string_literal: true
+
+class AddIdentifierToPrometheusMetric < ActiveRecord::Migration
+  include Gitlab::Database::MigrationHelpers
+
+  DOWNTIME = false
+
+  disable_ddl_transaction!
+
+  def up
+    add_concurrent_index :prometheus_metrics, :identifier, unique: true
+  end
+
+  def down
+    remove_concurrent_index :prometheus_metrics, :identifier, unique: true
+  end
+end

db/schema.rb

@@ -11,7 +11,7 @@
 #
 # It's strongly recommended that you check this file into your version control system.
 
-ActiveRecord::Schema.define(version: 20180831164909) do
+ActiveRecord::Schema.define(version: 20180831164910) do
 
   # These are extensions that must be enabled in order to support this database
   enable_extension "plpgsql"