Yeah, I know that indexes for unlogged tables are unlogged. I was just wondering if you could do this for logged tables. (Safely, such that on crash recovery WAL replay won't throw up, these can be omitted from base backups, etc.)
No, you cannot create unlogged indexes on logged tables. An unlogged tables is truncated when a server starts after a crash, and so are its indexes that become empty by default. But having an unlogged index on a logged table would mean that
you would need to truncate and regenerate the index after a crash as the data of the normal table is still here, what would impact the performance boot of the server. Do you have a particular use-case in mind? I cannot see advantages directly advantages in having an unlogged index on a logged table...