I am wondering if we should take the current SQL used by vacuumdb to find missing stats and perform direct syscache lookups in C?
So....about that. The exiting missing-stats-only queries test for a corresponding pg_statistic_ext_data row for any pg_statistic_ext row that meets the relation filters, but at this very moment we can restore all types of extended stats _except_ expressions. That functionality could make it into 19, but if it doesn't we're going to have to adjust vacuumdb to probe pg_statistic_ext.stxkeys for expression indexes and look for matching stxdexprs elements. I agree that those matches are better done with syscache lookups, but the SQL that we're treating as a spec might be a moving target in the near future.
