Add updated_at index to improve Merge Request API performance

db/post_migrate/20210722055217_add_updated_at_index_on_merge_requests.rb

+# frozen_string_literal: true
+
+class AddUpdatedAtIndexOnMergeRequests < ActiveRecord::Migration[6.1]
+  include Gitlab::Database::MigrationHelpers
+
+  INDEX_NAME = 'index_merge_requests_on_target_project_id_and_updated_at_and_id'
+
+  disable_ddl_transaction!
+
+  def up
+    add_concurrent_index :merge_requests, [:target_project_id, :updated_at, :id], name: INDEX_NAME
+  end
+
+  def down
+    remove_concurrent_index_by_name :merge_requests, INDEX_NAME
+  end
+end

db/schema_migrations/20210722055217

+bd934c20443d5a044caa9e92389018291ffb2bf60b8ca54d9baca4a0e70caf28
\ No newline at end of file

db/structure.sql

@@ -24179,6 +24179,8 @@ CREATE INDEX index_merge_requests_on_target_project_id_and_squash_commit_sha ON
 
 CREATE INDEX index_merge_requests_on_target_project_id_and_target_branch ON merge_requests USING btree (target_project_id, target_branch) WHERE ((state_id = 1) AND (merge_when_pipeline_succeeds = true));
 
+CREATE INDEX index_merge_requests_on_target_project_id_and_updated_at_and_id ON merge_requests USING btree (target_project_id, updated_at, id);
+
 CREATE INDEX index_merge_requests_on_target_project_id_iid_jira_description ON merge_requests USING btree (target_project_id, iid) WHERE (description ~ '[A-Z][A-Z_0-9]+-\d+'::text);
 
 CREATE INDEX index_merge_requests_on_title ON merge_requests USING btree (title);