We want all records from both tables (original + backup)
- UNION combines rows from both tables into a single result set
- JOIN would try to match records between tables (which isn't what we want)
- UNION ALL wouldn't eliminate duplicates, backup and original should have different data, so probably doesn't matter much, but I think it is safer to use UNION
For messages & medias, there is no select * query but more specialized queries; this is going to be handled as part of ENG-9787
Depends on D14845