Update MR index to include id

Updates an index used in the merge requests API endpoints
to cover all the fields being used in the query. This has
a notable performance benefit.

changelogs/unreleased/update-index-used-in-mr-api.yml

+---
+title: Update MR index to include id
+merge_request: 42222
+author:
+type: performance

db/migrate/20200914104642_modify_merge_request_api_index.rb

+# frozen_string_literal: true
+
+# See https://docs.gitlab.com/ee/development/migration_style_guide.html
+# for more information on how to write migrations for GitLab.
+
+class ModifyMergeRequestApiIndex < ActiveRecord::Migration[6.0]
+  include Gitlab::Database::MigrationHelpers
+
+  DOWNTIME = false
+
+  disable_ddl_transaction!
+
+  def up
+    add_concurrent_index :merge_requests, [:target_project_id, :created_at, :id]
+  end
+
+  def down
+    remove_concurrent_index :merge_requests, [:target_project_id, :created_at, :id]
+  end
+end

db/migrate/20200914105202_remove_old_merge_request_api_index.rb

+# frozen_string_literal: true
+
+class RemoveOldMergeRequestApiIndex < ActiveRecord::Migration[6.0]
+  include Gitlab::Database::MigrationHelpers
+
+  DOWNTIME = false
+
+  disable_ddl_transaction!
+
+  def up
+    remove_concurrent_index :merge_requests, [:target_project_id, :created_at]
+  end
+
+  def down
+    add_concurrent_index :merge_requests, [:target_project_id, :created_at]
+  end
+end

db/schema_migrations/20200914104642

+49b492717acca6d43e16caafda0b75f6e1a3630592149b04d1148758406ae1a7
\ No newline at end of file

db/schema_migrations/20200914105202

+6cd644220e07f477a87ddc8bb8b2de3ed920c0c76383122bab95c939e21abbb2
\ No newline at end of file

db/structure.sql

@@ -20326,6 +20326,8 @@ CREATE INDEX index_merge_requests_on_sprint_id ON public.merge_requests USING bt
 
 CREATE INDEX index_merge_requests_on_target_branch ON public.merge_requests USING btree (target_branch);
 
+CREATE INDEX index_merge_requests_on_target_project_id_and_created_at_and_id ON public.merge_requests USING btree (target_project_id, created_at, id);
+
 CREATE UNIQUE INDEX index_merge_requests_on_target_project_id_and_iid ON public.merge_requests USING btree (target_project_id, iid);
 
 CREATE INDEX index_merge_requests_on_target_project_id_and_target_branch ON public.merge_requests USING btree (target_project_id, target_branch) WHERE ((state_id = 1) AND (merge_when_pipeline_succeeds = true));
@@ -20338,8 +20340,6 @@ CREATE INDEX index_merge_requests_on_tp_id_and_merge_commit_sha_and_id ON public
 
 CREATE INDEX index_merge_requests_on_updated_by_id ON public.merge_requests USING btree (updated_by_id) WHERE (updated_by_id IS NOT NULL);
 
-CREATE INDEX index_merge_requests_target_project_id_created_at ON public.merge_requests USING btree (target_project_id, created_at);
-
 CREATE UNIQUE INDEX index_merge_trains_on_merge_request_id ON public.merge_trains USING btree (merge_request_id);
 
 CREATE INDEX index_merge_trains_on_pipeline_id ON public.merge_trains USING btree (pipeline_id);