Add a unique index to stages on pipeline / position

db/migrate/20180530135500_add_index_to_stages_position.rb

+class AddIndexToStagesPosition < ActiveRecord::Migration
+  include Gitlab::Database::MigrationHelpers
+
+  DOWNTIME = false
+
+  disable_ddl_transaction!
+
+  def up
+    add_concurrent_index :ci_stages, [:pipeline_id, :position], unique: true
+  end
+
+  def down
+    remove_concurrent_index :ci_stages, [:pipeline_id, :position]
+  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: 20180529093006) do
+ActiveRecord::Schema.define(version: 20180530135500) do
 
   # These are extensions that must be enabled in order to support this database
   enable_extension "plpgsql"
@@ -518,6 +518,7 @@ ActiveRecord::Schema.define(version: 20180529093006) do
   end
 
   add_index "ci_stages", ["pipeline_id", "name"], name: "index_ci_stages_on_pipeline_id_and_name", unique: true, using: :btree
+  add_index "ci_stages", ["pipeline_id", "position"], name: "index_ci_stages_on_pipeline_id_and_position", unique: true, using: :btree
   add_index "ci_stages", ["pipeline_id"], name: "index_ci_stages_on_pipeline_id", using: :btree
   add_index "ci_stages", ["project_id"], name: "index_ci_stages_on_project_id", using: :btree