The Wiki Replicas can be used for SQL queries. You can use Quarry, PAWS or other ways to do this. To make the Wiki Replicas stable there will be two changes. Cross-database JOINS will no longer work. You can also only query a database if you connect to it directly. This will happen in February 2021. If you think this affects you and you need help you can post on Phabricator or on Wikitech.