Add index of merge_request_id and rule_type

Adds an index on these 2 columns ahead of removing a similar index on
merge_request_id and code_owner, as the code_owner column is slated for
removal

changelogs/unreleased/230835-add-index-of-merge_request_id-on-approval-merge-request-rules-tabl.yml

+---
+title: Add index on merge_request_id to approval_merge_request_rules
+merge_request: 40556
+author:
+type: other

db/migrate/20200826212800_add_index_on_merge_request_id_and_rule_type_to_approval_merge_request_rule.rb

+# frozen_string_literal: true
+
+class AddIndexOnMergeRequestIdAndRuleTypeToApprovalMergeRequestRule < ActiveRecord::Migration[6.0]
+  include Gitlab::Database::MigrationHelpers
+
+  DOWNTIME = false
+
+  disable_ddl_transaction!
+
+  INDEX_NAME = "approval_mr_rule_index_merge_request_id"
+
+  def up
+    add_concurrent_index(
+      :approval_merge_request_rules,
+      :merge_request_id,
+      name: INDEX_NAME
+    )
+  end
+
+  def down
+    remove_concurrent_index_by_name :approval_merge_request_rules, INDEX_NAME
+  end
+end

db/schema_migrations/20200826212800

+360c42f4d34c3b03e7a0375a0ff2776f066888f0a40131180bf301b876ea58db
\ No newline at end of file

db/structure.sql

@@ -18922,6 +18922,8 @@ CREATE UNIQUE INDEX any_approver_merge_request_rule_type_unique_index ON public.
 
 CREATE UNIQUE INDEX any_approver_project_rule_type_unique_index ON public.approval_project_rules USING btree (project_id) WHERE (rule_type = 3);
 
+CREATE INDEX approval_mr_rule_index_merge_request_id ON public.approval_merge_request_rules USING btree (merge_request_id);
+
 CREATE UNIQUE INDEX approval_rule_name_index_for_code_owners ON public.approval_merge_request_rules USING btree (merge_request_id, code_owner, name) WHERE ((code_owner = true) AND (section IS NULL));
 
 CREATE UNIQUE INDEX backup_labels_group_id_project_id_title_idx ON public.backup_labels USING btree (group_id, project_id, title);