Add partial index on members to optimize highest access level query

changelogs/unreleased/327593-optimize-user-current-highest-access-level.yml

+---
+title: Add partial index on members to optimize highest access level query
+merge_request: 59455
+author:
+type: performance

db/post_migrate/20210415074645_index_members_on_user_id_access_level_requested_at_is_null.rb

+# frozen_string_literal: true
+
+class IndexMembersOnUserIdAccessLevelRequestedAtIsNull < ActiveRecord::Migration[6.0]
+  include Gitlab::Database::MigrationHelpers
+
+  disable_ddl_transaction!
+
+  INDEX_NAME = 'index_members_on_user_id_and_access_level_requested_at_is_null'
+
+  def up
+    add_concurrent_index(:members, [:user_id, :access_level], where: 'requested_at IS NULL', name: INDEX_NAME)
+  end
+
+  def down
+    remove_concurrent_index_by_name(:members, INDEX_NAME)
+  end
+end

db/schema_migrations/20210415074645

+393a7f1029f05f73c26e031c8980734e13c304846c3457d581c006dc2566d94d
\ No newline at end of file

db/structure.sql

@@ -23033,6 +23033,8 @@ CREATE INDEX index_members_on_requested_at ON members USING btree (requested_at)
 
 CREATE INDEX index_members_on_source_id_and_source_type ON members USING btree (source_id, source_type);
 
+CREATE INDEX index_members_on_user_id_and_access_level_requested_at_is_null ON members USING btree (user_id, access_level) WHERE (requested_at IS NULL);
+
 CREATE INDEX index_members_on_user_id_created_at ON members USING btree (user_id, created_at) WHERE ((ldap = true) AND ((type)::text = 'GroupMember'::text) AND ((source_type)::text = 'Namespace'::text));
 
 CREATE INDEX index_members_on_user_id_source_id_source_type ON members USING btree (user_id, source_id, source_type);