It's a good idea to revisit, but I think I need SELSERT or something. Are you saying the `DO NOTHING` clause can still result in a row being returned with `RETURNING *`? My impression is that it can't but I'd be excited to be wrong!
edit: Ah, I bet the implicit suggestion was to do `INSERT ... ON CONFLICT DO NOTHING RETURNING *` and if no row was returned, you hope to know/guess the conflict and do an extra `SELECT`. I'll think more on this, thanks.