Optimize query for cherry picked merge requests

This commit optimizes the query for seraching
cherry picked merge requests through `notes` table.

changelogs/unreleased/optimize-query-for-cherry-picked-merge-requests.yml

+---
+title: Optimize query for cherry picked merge requests
+merge_request: 58967
+author:
+type: performance

db/migrate/20210409084242_create_index_on_notes_for_cherry_picked_merge_requests.rb

+# frozen_string_literal: true
+
+class CreateIndexOnNotesForCherryPickedMergeRequests < ActiveRecord::Migration[6.0]
+  include Gitlab::Database::MigrationHelpers
+
+  DOWNTIME = false
+  NAME = 'index_notes_for_cherry_picked_merge_requests'
+
+  disable_ddl_transaction!
+
+  def up
+    add_concurrent_index :notes, [:project_id, :commit_id], where: "((noteable_type)::text = 'MergeRequest'::text)", name: NAME
+  end
+
+  def down
+    remove_concurrent_index_by_name :notes, name: NAME
+  end
+end

db/schema_migrations/20210409084242

+6bd35117ca922fc0d9cb8cbd9b0e6d5d9216457182d5679e705c1f03eef05921
\ No newline at end of file

db/structure.sql

@@ -23207,6 +23207,8 @@ CREATE INDEX index_non_requested_project_members_on_source_id_and_type ON member
 
 CREATE UNIQUE INDEX index_note_diff_files_on_diff_note_id ON note_diff_files USING btree (diff_note_id);
 
+CREATE INDEX index_notes_for_cherry_picked_merge_requests ON notes USING btree (project_id, commit_id) WHERE ((noteable_type)::text = 'MergeRequest'::text);
+
 CREATE INDEX index_notes_on_author_id_and_created_at_and_id ON notes USING btree (author_id, created_at, id);
 
 CREATE INDEX index_notes_on_commit_id ON notes USING btree (commit_id);