TRUNCATE funziona, grazie. 
. Ora controllerò se il backup è andato a buon fine.
EDIT:
Oh no 
!
[2020-05-17 16:56:39] pg_dump: Dumping the contents of table "post_search_data" failed: PQgetResult() failed.
[2020-05-17 16:56:39] pg_dump: Error message from server: ERROR: invalid page in block 89 of relation base/16384/22213
[2020-05-17 16:56:39] pg_dump: The command was: COPY public.post_search_data (post_id, search_data, raw_data, locale, version) TO stdout;
[2020-05-17 16:56:39] Unpausing sidekiq...
[2020-05-17 16:56:39] EXCEPTION: pg_dump failed
[2020-05-17 16:56:39] /var/www/discourse/lib/backup_restore/backuper.rb:183:in `dump_public_schema'
EDIT 2 
Risolto utilizzando https://stackoverflow.com/questions/5220344/postgresql-invalid-page-header-in-block.