Peter Thornton via Mailman-users writes:
We have a legacy environment with: there are two sqlite3 dbs for mailman core and mailman web.
I've staged a new environment there is a single postgres db here for both core and web.
This is known to be problematic. Try searching the archives https://lists.mailman3.org/archives/list/mailman-users@mailman3.org/ for "sqlite postgres migration" (or maybe use "sqlite3" and/or "postgresql", and don't surround the phrase with quotes).
I would start by configuring the postgres db on the new host with separate databases. If you decide you want a single db, I guess it's easier to add the tables from the mailmanweb db to the mailman db than it is to get pgloader to do the right thing.
ISTR that the most straightforward approach was rather than using pgloader, you should create the databases in Postgres then dump the databases (text) and load them. You probably need to edit them. (This is very unreliable advice!)
It's almost 2am here, so I'm off to bed. You should consider all of the above advice except "known problematic" to be unreliable.
-- GNU Mailman consultant (installation, migration, customization) Sirius Open Source https://www.siriusopensource.com/ Software systems consulting in Europe, North America, and Japan