Index merge requests with state_id

This adds an additional index to cover an expensive query
used on the merge_requests index.

changelogs/unreleased/mr-state-index.yml

+---
+title: Add state_id index for merge_requests list
+merge_request: 42481
+author:
+type: performance

db/migrate/20200916135044_add_state_id_index_to_merge_requests.rb

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

db/schema_migrations/20200916135044

+299e6fb38da303f06ab006a25701ff1c58ce2abfa8cf93c3c8b866ee0b2bd1cc
\ No newline at end of file

db/structure.sql

@@ -20486,6 +20486,8 @@ CREATE INDEX index_merge_requests_on_target_project_id_and_created_at_and_id ON
 
 CREATE UNIQUE INDEX index_merge_requests_on_target_project_id_and_iid ON merge_requests USING btree (target_project_id, iid);
 
+CREATE INDEX index_merge_requests_on_target_project_id_and_iid_and_state_id ON merge_requests USING btree (target_project_id, iid, state_id);
+
 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_title ON merge_requests USING btree (title);