What's the simplest way to snapshot a database, perform an operation in app, snapshot it again and get an overview of how many rows were affected in each table? Like an overview of diff. Optionally list changed pkey ids. I tried googling but couldn't find anything like this, so I was thinking of making a DYI solution but wasn't that desperate yet.
So you could effectively:
1) Create a snapshot of your production DB -> DB_2A
2) Then create a snapshot of that snapshot -> DB_2B
3) Now you have two copies of the exact same database. Run your query/workload/migration on DB_2B.
4) Run some metadata queries against DB_2A and DB_2B and compare the results.
5) If your metadata queries are inline with expectations, delete the snapshots. If not, leave them around for a bit for manual inspection.
If you have multiple DB snapshots and WAL enabled, theoretically you could inspect the log to see what happened inbetween. There's pg_xlogdump for that but I think it will output very raw data...