Add index for head pipeline id

db/migrate/20170619144837_add_index_for_head_pipeline_merge_request.rb

+class AddIndexForHeadPipelineMergeRequest < ActiveRecord::Migration
+  include Gitlab::Database::MigrationHelpers
+
+  DOWNTIME = false
+
+  disable_ddl_transaction!
+
+  def up
+    add_concurrent_index :merge_requests, :head_pipeline_id
+  end
+
+  def down
+    remove_concurrent_index :merge_requests, :head_pipeline_id if index_exists?(:merge_requests, :head_pipeline_id)
+  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: 20170614115405) do
+ActiveRecord::Schema.define(version: 20170619144837) do
 
   # These are extensions that must be enabled in order to support this database
   enable_extension "plpgsql"
@@ -777,6 +777,7 @@ ActiveRecord::Schema.define(version: 20170614115405) do
   add_index "merge_requests", ["created_at"], name: "index_merge_requests_on_created_at", using: :btree
   add_index "merge_requests", ["deleted_at"], name: "index_merge_requests_on_deleted_at", using: :btree
   add_index "merge_requests", ["description"], name: "index_merge_requests_on_description_trigram", using: :gin, opclasses: {"description"=>"gin_trgm_ops"}
+  add_index "merge_requests", ["head_pipeline_id"], name: "index_merge_requests_on_head_pipeline_id", using: :btree
   add_index "merge_requests", ["milestone_id"], name: "index_merge_requests_on_milestone_id", using: :btree
   add_index "merge_requests", ["source_branch"], name: "index_merge_requests_on_source_branch", using: :btree
   add_index "merge_requests", ["source_project_id"], name: "index_merge_requests_on_source_project_id", using: :btree