Add index for ci_commits for gl_project_id, ref, status

and remove the old one which we don't really need.

db/migrate/20170216135621_add_index_for_latest_successful_pipeline.rb

+class AddIndexForLatestSuccessfulPipeline < ActiveRecord::Migration
+  include Gitlab::Database::MigrationHelpers
+  DOWNTIME = false
+
+  disable_ddl_transaction!
+
+  def change
+    add_concurrent_index(:ci_commits, [:gl_project_id, :ref, :status])
+  end
+end

db/migrate/20170216141440_drop_index_for_builds_project_status.rb

+class DropIndexForBuildsProjectStatus < ActiveRecord::Migration
+  include Gitlab::Database::MigrationHelpers
+  DOWNTIME = false
+
+  def change
+    remove_index(:ci_commits, [:gl_project_id, :status])
+  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: 20170215200045) do
+ActiveRecord::Schema.define(version: 20170216141440) do
 
   # These are extensions that must be enabled in order to support this database
   enable_extension "plpgsql"
@@ -251,8 +251,8 @@ ActiveRecord::Schema.define(version: 20170215200045) do
     t.integer "lock_version"
   end
 
+  add_index "ci_commits", ["gl_project_id", "ref", "status"], name: "index_ci_commits_on_gl_project_id_and_ref_and_status", using: :btree
   add_index "ci_commits", ["gl_project_id", "sha"], name: "index_ci_commits_on_gl_project_id_and_sha", using: :btree
-  add_index "ci_commits", ["gl_project_id", "status"], name: "index_ci_commits_on_gl_project_id_and_status", using: :btree
   add_index "ci_commits", ["gl_project_id"], name: "index_ci_commits_on_gl_project_id", using: :btree
   add_index "ci_commits", ["status"], name: "index_ci_commits_on_status", using: :btree
   add_index "ci_commits", ["user_id"], name: "index_ci_commits_on_user_id", using: :btree