Add an unique index on pipeline stage name

db/post_migrate/20180125111139_add_unique_pipeline_stage_name_index.rb

+class AddUniquePipelineStageNameIndex < ActiveRecord::Migration
+  include Gitlab::Database::MigrationHelpers
+
+  DOWNTIME = false
+
+  disable_ddl_transaction!
+
+  def up
+    remove_concurrent_index :ci_stages, [:pipeline_id, :name]
+    add_concurrent_index :ci_stages, [:pipeline_id, :name], unique: true
+  end
+
+  def down
+    remove_concurrent_index :ci_stages, [:pipeline_id, :name], unique: true
+    add_concurrent_index :ci_stages, [:pipeline_id, :name]
+  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: 20180119121225) do
+ActiveRecord::Schema.define(version: 20180125111139) do
   # These are extensions that must be enabled in order to support this database
   enable_extension "plpgsql"
@@ -450,7 +450,8 @@ ActiveRecord::Schema.define(version: 20180119121225) do
     t.integer "lock_version"
   end
-  add_index "ci_stages", ["pipeline_id", "name"], name: "index_ci_stages_on_pipeline_id_and_name", using: :btree
+  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", "name"], name: "index_ci_stages_on_pipeline_id_and_name_unique", 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